Light-emitting element, light-emitting device and electronic device

ABSTRACT

The light-emitting element of the present invention includes a light-emitting layer and a layer for controlling movement of carriers between a pair of electrodes. The layer for controlling movement of carriers includes a first organic compound having a carrier transporting property and a second organic compound for reducing the carrier transporting property of the first organic compound, and the second organic compound is dispersed in the first organic compound. The layer for controlling movement of carriers is provided in such a manner, whereby change in carrier balance with time can be suppressed. Therefore, a light-emitting element having a long lifetime can be obtained.

This application is a continuation of copending application Ser. No.13/025,578 filed on Feb. 11, 2011 which is a continuation of applicationSer. No. 11/821,757 filed on Jun. 25, 2007 (now U.S. Pat. No. 7,902,742issued Mar. 8, 2011).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to light-emitting elements of a currentexcitation type. The present invention also relates to light-emittingdevices and electronic devices each of which has the light-emittingdevice. More specifically, the invention relates to light-emittingelements of a current excitation type having a long lifetime. Further,the invention relates to light-emitting devices and electronic deviceseach of which uses the light-emitting device.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence. As abasic structure of these light-emitting elements, a structure where asubstance having light-emitting property is interposed between a pair ofelectrodes is used. By application of a voltage to this element, lightemission from a substance having light-emitting property can beobtained.

Since such a light-emitting element is a self-luminous type, there areadvantages such as higher visibility of a pixel than visibility of aliquid crystal display, and unnecessity of a backlight. Accordingly,such a light-emitting element is considered to be suitable as a flatpanel display element. In addition, such a light-emitting element can bemanufactured to be thin and light, which is a great advantage. Moreover,the light-emitting element has a feature that response speed isextremely fast.

Furthermore, since such a light-emitting element can be formed into afilm form, planar light emission can be easily obtained by formation ofa large-area element. This characteristic is difficult to be obtained bya point light source typified by an incandescent lamp or an LED, or aline light source typified by a fluorescent lamp. Therefore, thelight-emitting element has a high utility value as a plane light sourcethat can be applied to lighting or the like.

The light-emitting elements using electroluminescence are classifiedroughly in accordance with whether they use an organic compound or aninorganic compound as a substance having light-emitting property. In thepresent invention, a substance having light-emitting property is anorganic compound.

In a case where the substance having light-emitting property is anorganic compound, by application of voltage to the light-emittingelement, electrons and holes are injected from the pair of electrodesinto the layer including an organic compound having light-emittingproperty to cause current flow. Then, by recombination of these carriers(electrons and holes), the organic compound having light-emittingproperty gets in an excited state, and light is emitted when the excitedstate returns to a ground state.

Because of such a mechanism, this kind of light-emitting element isreferred to as a light-emitting element of a current excitation type. Itis to be noted that an excited state formed by an organic compound canbe a singlet excited state or a triplet excited state. Light emissionfrom the singlet excited state is referred to as fluorescence, and lightemission from the triplet excited state is referred to asphosphorescence.

In order to overcome many problems derived from materials of such alight-emitting element and to improve its element characteristics,improvement in an element structure, material development, and so on arecarried out.

For example, in Non-Patent Document 1, a hole blocking layer is providedso that a light-emitting element using a phosphorescent materialefficiently emits light.

NON-PATENT DOCUMENT

-   Tetsuo TSUTSUI et al., Japanese Journal of Applied Physics, vol. 38,    L1502-L1504 (1999)

SUMMARY OF THE INVENTION

However, as disclosed in the Non-Patent Document, the hole blockinglayer has poor durability, and the light emitting element has a shortlifetime. Therefore, the light emitting element is desired to have along lifetime. In view of the foregoing problem, it is an object of thepresent invention to provide a light-emitting element having a longlifetime. Further, it is another object of the present invention toprovide a light-emitting device and an electronic device each having along lifetime.

As a result of diligent study, the present inventors have found thatwhen a layer for controlling movement of carriers is provided, changewith time of carrier balance can be suppressed. Thus, the presentinventors have also found that a light-emitting element having longlifetime can be obtained.

Therefore, one aspect of the present invention is a light-emittingelement including a light-emitting layer and a layer for controllingmovement of carriers between a pair of electrodes, where the layer forcontrolling movement of carriers includes a first organic compoundhaving a carrier transporting property and a second organic compound forreducing the carrier transporting property of the first organiccompound, and the second organic compound is dispersed in the firstorganic compound.

In the above structure, when the carrier is an electron, the differencebetween the lowest unoccupied molecular orbital level of the firstorganic compound and the lowest unoccupied molecular orbital level ofthe second organic compound is preferably less than 0.3 eV. In addition,in the above structure, when the carrier is an electron, the firstorganic compound is preferably a metal complex, and the second organiccompound is preferably an aromatic amine compound. Further, in the abovestructure, when the carrier is a hole, the difference between thehighest occupied molecular orbital level of the first organic compoundand the highest occupied molecular orbital level of the second compoundis preferably less than 0.3 eV. Furthermore, in the above structure,when the carrier is a hole, the first organic compound is preferably anaromatic amine compound, and the second organic compound is preferably ametal complex.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and two layers for controlling movementof carriers between a first electrode and a second electrode. One of thelayers for controlling movement of carriers is provided between thelight-emitting layer and the second electrode, and the other layer forcontrolling movement of carriers is provided between the light-emittinglayer and the first electrode. The one of the layers for controllingmovement of carriers includes a first organic compound having a carriertransporting property and a second organic compound for reducing thecarrier transporting property of the first organic compound, and thesecond organic compound is dispersed in the first organic compound. Theother layer for controlling movement of carriers includes a thirdorganic compound having a carrier transporting property and a fourthorganic compound for reducing the carrier transporting property of thethird organic compound, and the fourth organic compound is dispersed inthe third organic compound. The carrier transporting property of the oneof the layers for controlling movement of carriers is different from thecarrier transporting property of the other layer for controllingmovement of carriers.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a pair of electrodes, where the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, and the first organic compound and the second organiccompound have transport carriers of different polarity from each other.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a pair of electrodes, where the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, the first organic compound is an organic compoundhaving electron transporting property, and the second organic compoundis an organic compound having hole transporting property.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a first electrode and a second electrode, where thelayer for controlling movement of carriers is provided between thelight-emitting layer and the second electrode, the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, the first organic compound is an organic compoundhaving electron transporting property, the second organic compound is anorganic compound having hole transporting property, in the layer forcontrolling movement of carriers, the proportion of weight of the firstorganic compound is greater than the proportion of weight of the secondorganic compound, and when voltage is applied so that potential of thefirst electrode is higher than potential of the second electrode, lightemission from the light-emitting layer can be obtained.

In the above structure, the difference between the lowest unoccupiedmolecular orbital level of the first organic compound and the lowestunoccupied molecular orbital level of the second organic compound ispreferably less than 0.3 eV. In addition, the first organic compound ispreferably a metal complex, and the second organic compound ispreferably an aromatic amine compound. Further, in the above structure,the light-emitting layer preferably has electron transporting property.Alternatively, it is preferable that the light-emitting layer include athird organic compound and a fourth organic compound, the proportion ofweight of the third organic compound be higher than the proportion ofweight of fourth organic compound, and the third organic compound haveelectron transporting property. In this case, the structures of thefirst organic compound and the third organic compound are preferablydifferent organic compounds from each other.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a first electrode and a second electrode, where thelayer for controlling movement of carriers is provided between thelight-emitting layer and the first electrode, the layer for controllingmovement of carriers includes a rust organic compound and a secondorganic compound, the first organic compound is an organic compoundhaving hole transporting property, the second organic compound is anorganic compound having electron transporting property, in the layer forcontrolling movement of carriers, the proportion of weight of the firstorganic compound is greater than the proportion of weight of the secondorganic compound, and when voltage is applied so that potential of thefirst electrode is higher than potential of the second electrode, lightemission from the light-emitting layer can be obtained.

In the above structure, the difference between the highest occupiedmolecular orbital level of the first organic compound and the highestoccupied molecular orbital level of the second organic compound ispreferably less than 0.3 eV. Further, the first organic compound ispreferably an aromatic amine compound, and the second organic compoundis preferably a metal complex. In addition, in the above structure, thelight-emitting layer preferably has hole transporting property.Alternatively, it is preferable that the light-emitting layer include athird organic compound and a fourth organic compound, the proportion ofweight of the third organic compound be higher than the proportion ofweight of the fourth organic compound, and the third organic compoundhave hole transporting property. In this case, the structures of thefirst organic compound and the third organic compound are preferablydifferent organic compounds from each other.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and two layers for controlling movementof carriers between a first electrode and a second electrode. One of thelayers for controlling movement of carriers is provided between thelight-emitting layer and the second electrode, and the other layer forcontrolling movement of carriers is provided between the light-emittinglayer and the first electrode. The one of the layers for controllingmovement of carriers includes a first organic compound having electrontransporting property and a second organic compound having holetransporting property, and in the one of the layers for controllingmovement of carriers, the proportion of weight of the first organiccompound is greater than the proportion of weight of the second organiccompound. The other layer for controlling movement of carriers includesa third organic compound having hole transporting property and a fourthorganic compound having electron transporting property, and in the otherlayer for controlling movement of carriers, the proportion of weight ofthe third organic compound is greater than the proportion of weight ofthe fourth organic compound. When voltage is applied so that potentialof the first electrode is higher than potential of the second electrode,light emission from the light-emitting layer can be obtained.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a pair of electrodes, where the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, and when the magnitude of the dipole moment of thefirst organic compound is P₁ and the magnitude of the dipole moment ofthe second organic compound is P₂, a relation of P₁/P₂≧3 or P₁/P₂≦0.33is satisfied.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a first electrode and a second electrode, where thelayer for controlling movement of carriers is provided between thelight-emitting layer and the second electrode, the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, when the magnitude of the dipole moment of the firstorganic compound is P₁ and the magnitude of the dipole moment of thesecond organic compound is P₂, P₁/P₂≧3 is satisfied, in the layer forcontrolling movement of carriers, the proportion of weight of the firstorganic compound is greater than the proportion of weight of the secondcompound, and when voltage is applied so that potential of the firstelectrode is higher than potential of the second electrode, lightemission from the light-emitting layer can be obtained.

In the above structure, the difference between the lowest unoccupiedmolecular orbital level of the first organic compound and the lowestunoccupied molecular orbital level of the second organic compound ispreferably less than 0.3 eV. Further, the first organic compound ispreferably a metal complex, and the second organic compound ispreferably an aromatic amine compound. In addition, in the abovestructure, the light-emitting layer preferably has electron transportingproperty. Alternatively, it is preferable that the light-emitting layerinclude a third organic compound and a fourth organic compound, theproportion of weight of the third organic compound be higher than theproportion of weight of the fourth organic compound, and the thirdorganic compound have electron transporting property. In this case, thestructures of the first organic compound and the third organic compoundare preferably different organic compounds from each other.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and a layer for controlling movement ofcarriers between a first electrode and a second electrode, where thelayer for controlling movement of carriers is provided between thelight-emitting layer and the first electrode, the layer for controllingmovement of carriers includes a first organic compound and a secondorganic compound, when the magnitude of the dipole moment of the firstorganic compound is P₁ and the magnitude of the dipole moment of thesecond organic compound is P₂, P₁/P₂≦0.33 is satisfied, in the layer forcontrolling movement of carriers, the proportion of weight of the firstorganic compound is greater than the proportion of weight of the secondorganic compound, and when voltage is applied so that potential of thefirst electrode is higher than potential of the second electrode, lightemission from the light-emitting layer can be obtained.

In the above structure, the difference between the highest occupiedmolecular orbital level of the first organic compound and the highestoccupied molecular orbital level of the second organic compound ispreferably less than 0.3 eV. Further, the first organic compound ispreferably an aromatic amine compound, and the second organic compoundis preferably a metal complex. In addition, in the above structure, thelight-emitting layer preferably has hole transporting property.Alternatively, it is preferable that the light-emitting layer include athird organic compound and a fourth organic compound, the proportion ofweight of the third organic compound be higher than the proportion ofweight of the fourth organic compound, and the third organic compoundhave hole transporting property. In this case, the structures of thefirst organic compound and the third organic compound are preferablydifferent organic compounds from each other.

Another aspect of the present invention is a light-emitting elementincluding a light-emitting layer and two layers for controlling movementof carriers between a first electrode and a second electrode. One of thelayers for controlling movement of carriers is provided between thelight-emitting layer and the second electrode, and the other layer forcontrolling movement of carriers is provided between the light-emittinglayer and the first electrode. The one of the layers for controllingmovement of carriers includes a first organic compound and a secondorganic compound; when the magnitude of the dipole moment of the firstorganic compound is P₁ and the magnitude of the dipole moment of thesecond organic compound is P₂, P₁/P₂≧3 is satisfied; and in the one ofthe layers for controlling movement of carriers, the proportion ofweight of the first organic compound is greater than the proportion ofweight of the second organic compound. The other layer for controllingmovement of carriers includes a third organic compound and a fourthorganic compound; when the magnitude of the dipole moment of the thirdorganic compound is P₃ and dipole moment of the fourth organic compoundis P₄, P₃/P₄≦0.33 is satisfied; and in the other layer for controllingmovement of carriers, the proportion of weight of the third organiccompound is greater than the proportion of weight of the fourth organiccompound. When voltage is applied so that potential of the firstelectrode is higher than potential of the second electrode, lightemission from the light-emitting layer can be obtained.

In the above structure, a thickness of the layer for controllingmovement of carriers is preferably greater than or equal to 5 nm andless than or equal to 20 nm. In addition, in the above structure, thelayer for controlling movement of carriers and the light-emitting layerare preferably provided to be in contact with each other.

Further, the present invention includes a light-emitting device havingthe above-described light-emitting element in its category. Thelight-emitting device in the present specification includes an imagedisplay device, a light-emitting device, or a light source (including alighting device). In addition, the light-emitting device of the presentinvention includes all the following modules: a module in which aconnector such as an FPC (flexible printed circuit), a TAB (tapeautomated bonding) tape or a TCP (tape carrier package) is attached to apanel provided with a light-emitting element; a module provided with aprinted wiring board at the end of the TAB tape or the TCP; and a modulein which an IC (integrated circuit) is directly mounted by a COG (chipon glass) method to a substrate on which a light-emitting element isformed.

Furthermore, the present invention includes an electronic device usingthe light-emitting element of the present invention for the displayportion in its category. Accordingly, one feature of the electronicdevice of the present invention is to include a display portion providedwith the above-described light-emitting element and a controller forcontrolling light emission of the light-emitting element.

In a light-emitting element of the present invention, a layer forcontrolling movement of carriers is provided, and change with time ofcarrier balance can be suppressed. Therefore, a light-emitting elementhaving a long lifetime can be obtained. Further, the light-emittingelement of the present invention is applied to a light-emitting deviceand an electronic device, whereby a light-emitting device and anelectronic device each of which has high luminous efficiency and reducedpower consumption can be obtained. In addition, a light-emitting deviceand an electronic device each of which has a long lifetime can beobtained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are views showing a light-emitting element in EmbodimentMode 1 of the present invention.

FIGS. 2A and 2B are views showing a light-emitting element that has adifferent stacked structure from that of FIGS. 1A and 1B in EmbodimentMode 1 of the present invention.

FIGS. 3A to 3C are views showing a light emission mode of alight-emitting element in Embodiment Mode 1 of the present invention.

FIG. 4 is a view showing a concept of a layer for controlling movementof carriers in a light-emitting element in Embodiment Mode 1 of thepresent invention.

FIGS. 5A and 5B are views showing a light-emitting element in EmbodimentMode 1 of the present invention.

FIGS. 6A and 6B are views showing a light-emitting element that has adifferent stacked structure from that of FIGS. 5A and 5B in EmbodimentMode 2 of the present invention.

FIGS. 7A to 7C are views showing a light emission mode of alight-emitting element in Embodiment Mode 2 of the present invention.

FIG. 8 is a view showing a concept of a layer for controlling movementof carriers in a light-emitting element in Embodiment Mode 2 of thepresent invention.

FIG. 9 is a view showing a light-emitting element in which a pluralityof light-emitting units are stacked in Embodiment mode 3 of the presentinvention.

FIGS. 10A and 10B are views showing an active matrix light-emittingdevice in Embodiment Mode 4 of the present invention.

FIGS. 11A and 11B are views showing a passive matrix light-emittingdevice in Embodiment Mode 4 of the present invention.

FIGS. 12A to 12D are views illustrating electronic devices of thepresent invention.

FIG. 13 is a view showing an electronic device in which a light-emittingdevice of the present invention is used as a backlight.

FIG. 14 is a view showing a table lamp as a lightning device of thepresent invention.

FIG. 15 is a view using an indoor lighting device as a lightning deviceof the present invention.

FIG. 16 is a view showing a light-emitting element of an embodiment.

FIG. 17 is a graph showing a current density vs. luminancecharacteristic of a light-emitting element 1.

FIG. 18 is a graph showing a voltage vs. luminance characteristic of alight-emitting element 1.

FIG. 19 is a graph showing a luminance vs. current efficiencycharacteristic of a light-emitting element 1.

FIG. 20 is a graph showing a light emission spectrum of a light-emittingelement 1.

FIG. 21 is a graph showing a result of a continuous lighting test byconstant current driving of a light-emitting element 1 and a referencelight-emitting element 2.

FIG. 22 is a graph showing a current density vs. luminancecharacteristic of a light-emitting element 3.

FIG. 23 is a graph showing a voltage vs. luminance characteristic of alight-emitting element 3.

FIG. 24 is a graph showing a luminance-current efficiency characteristicof a light-emitting element 3.

FIG. 25 is a graph showing a light emission spectrum of a light-emittingelement 3.

FIG. 26 is a graph showing a reductive reaction characteristic of Alq.

FIG. 27 is a graph showing a reductive reaction characteristic of2PCAPA.

FIG. 28 is a graph showing a reductive reaction of Coumarin 30.

DETAILED DESCRIPTION OF THE INVENTION

First, luminance decay factors of a light-emitting element will beexplained. A light-emitting element is normally driven by a constantcurrent, and in that case, luminance decay indicates reduction incurrent efficiency. Since current efficiency is a luminance of light pera unit current value, the current efficiency significantly depends onhow many flowing carriers contribute to recombination of carriers in alight-emitting layer (carrier balance) or how many carriers that arerecombined in the light-emitting layer (that is, exciton) contribute tolight emission (quantum yield).

Accordingly, it is considered that change in carrier balance with timeor time degradation of quantum yield is a main luminance decay factor.In the present invention, change in carrier balance with time isfocused.

Hereinafter, embodiment modes of the present invention will be explainedin detail with reference to drawings. However, the present invention isnot limited to explanation below, and the present invention is easilyunderstood by those skilled in the art that various changes andmodifications are possible, unless such changes and modifications departfrom the content and the scope of the invention. Therefore, the presentinvention is not construed as being limited to the description of thefollowing embodiment modes.

Embodiment Mode 1

A mode of a light-emitting element of the present invention will beexplained below with reference to FIG. 1A. In this embodiment mode, alight-emitting element provided with a layer for controlling movement ofelectrons as a layer for controlling movement of carriers will beexplained. That is, in the present invention, carriers are recombined ina portion away from the electrodes using the layer for controllingmovement of carriers in order to suppress change in carrier balance withtime and to make a light-emitting element have a long lifetime.

In a light-emitting element of the present invention, a plurality oflayers are provided between a pair of electrodes. The plurality oflayers are stacked by combining layers including a substance having ahigh carrier injecting property or a substrate having a high carriertransporting property so that a light-emitting region is formed in aportion away from the electrodes, in other words, carriers arerecombined in a portion away from the electrodes.

In this embodiment mode, the light-emitting element includes a firstelectrode 202, a second electrode 204, and an EL layer 203 providedbetween the first electrode 202 and the second electrode 204. It is tobe noted that this embodiment mode is explained below on the assumptionthat the first electrode 202 serves as an anode and the second electrode204 serves as a cathode. That is, this embodiment mode is explainedbelow on the assumption that, when a voltage is applied to the firstelectrode 202 and the second electrode 204 so that potential of thefirst electrode 202 is higher than potential of the second electrode204, light emission can be obtained.

A substrate 201 is used as a base of the light-emitting element. As thesubstrate 201, glass, plastic, or the like can be used, for example. Inthe light-emitting element of the present invention, the substrate 201may be left in a light-emitting device or an electronic device each ofwhich uses the light-emitting element. Further, the substrate 201 mayserve as a base in the manufacturing process of the light-emittingelement. In that case, the substrate 201 is not left in a manufacturedproduct ultimately.

As a material used for the first electrode 202, a metal, an alloy, anelectroconductive compound, a mixture thereof, or the like with a highwork function (specifically, a work function of 4.0 eV or higher) ispreferably used. Specifically, for example, indium oxide-tin oxide (ITO:Indium Tin Oxide), indium oxide-tin oxide containing silicon or siliconoxide, indium oxide-zinc oxide (IZO: Indium Zinc Oxide), indium oxidecontaining tungsten oxide and zinc oxide (IWZO), and the like can begiven.

Such conductive metal oxide films are usually formed by sputtering, butmay also be formed by ink-jet, spin coating, or the like by applicationof sol-gel method or the like. For example, indium oxide-zinc oxide(IZO) can be formed by sputtering using a target in which zinc oxide of1 to 20 wt % is added to indium oxide. Indium oxide containing tungstenoxide and zinc oxide (IWZO) can be formed by sputtering using a targetcontaining tungsten oxide of 0.5 to 5 wt % and zinc oxide of 0.1 to 1 wt% with respect to indium oxide. Other than these, gold (Au), platinum(Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron(Fe), cobalt (Co), copper (Cu), palladium (Pd), titanium (Ti), nitridesof the metal materials (such as titanium nitride: TiN), and the like canbe given.

In a case where a layer including a composite material described belowis used as a layer in contact with the first electrode, various metals,alloys, electroconductive compounds, and mixture thereof can be used asthe first electrode regardless of the work functions. For example,aluminum (Al), silver (Ag), an alloy containing aluminum (AlSi or thelike), or the like can be used. It is to be noted that the term“composition” indicates not just mixing two materials simply, but astate in which charges can be donated and received between materials bymixture of a plurality of materials.

Further, a metal of an element belonging to Group 1 or Group 2 in theperiodic table, which is a low work function material, that is, analkali metal such as lithium (Li) or cesium (Cs), an alkaline earthmetal such as magnesium (Mg), calcium (Ca), or strontium (Sr), an alloyincluding these metals (such as an MgAg alloy or an AlLi alloy), a rareearth metal such as europium (Eu) or ytterbium (Yb), an alloy includingsuch rare earth metals, or the like can be used. A film of an alkalimetal, an alkaline earth metal, or an alloy including these can beformed by vacuum evaporation. In addition, an alloy including an alkalimetal or an alkaline earth metal can be formed by a sputtering. Further,silver paste or the like can be formed by ink-jet.

The EL layer 203 has a first layer 211, a second layer 212, a thirdlayer 213, a fourth layer 214, a fifth layer 215, and a sixth layer 216.In this EL layer 203, the third layer 213 is a light-emitting layer, andthe fourth layer 214 is a layer for controlling movement of carriers. Itis acceptable as long as the EL layer 203 includes a layer forcontrolling movement of carriers shown in this embodiment mode and alight-emitting layer, and the other layers are not particularly limited.For example, the EL layer 203 can be formed by appropriate combinationof a hole injecting layer, a hole transporting layer, a light-emittinglayer, a layer for controlling movement of carriers, an electrontransporting layer, an electron injecting layer, and the like.

The first layer 211 is a layer including a substance having a high holeinjecting property. As the substance having a high hole injectingproperty, molybdenum oxide (MoOx), vanadium oxide (VOx), ruthenium oxide(RuOx), tungsten oxide (WOx), manganese oxide (MnOx), or the like can beused. In addition, as a low molecular organic compound, aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc), copper (II) phthalocyanine (abbreviation: CuPc), or vanadylphthalocyanine (VOPc) can be given.

Furthermore, aromatic amine compounds can be given, such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphtyl)-N-(9-phenylcarbazole-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). In addition, other compounds can be given.

As the first layer 211, a composite material in which an acceptorsubstance is mixed into a substance having a high hole transportingproperty can be used. It is to be noted that, by using the compositematerial in which an acceptor substance is mixed into a substance havinga high hole transporting property, a material used to form an electrodecan be selected regardless of its work function. In other words, besidesa material with a high work function, a material with a low workfunction may also be used as the first electrode 202. Such compositematerials can be formed by co-evaporation of a substance having a highhole transporting property and an acceptor substance.

As an organic compound used for the composite material, variouscompounds can be used, such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, and high molecular compounds (suchas oligomer, dendrimer, and polymer). As the organic compound used forthe composite material, an organic compound having a high holetransporting property is preferably used. Specifically, a substancehaving hole mobility of 10⁻⁶ cm²/Vs or higher is preferably used.However, other substances than those may also be used as long as thesubstances have higher hole transporting properties than electrontransporting properties. Organic compounds that can be used for thecomposite material will be specifically shown below.

As the organic compound that can be used for the composite material, thefollowing materials can be given, for example: aromatic amine compoundssuch as MTDATA, TDATA, DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); or carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Furthermore, the following aromatic hydrocarbon compounds can be given,such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butyl-anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracen,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

As an acceptor substance, organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil can be given. In addition, a transition metaloxide can be given. In addition, oxides of metals that belong to Group 4to Group 8 of the periodic table can be given. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause their electron accepting properties are high. Among these,molybdenum oxide is especially preferable because it is stable in theair and its hygroscopic property is low so that it can be easilytreated.

As the first layer 211, high molecular compounds (such as oligomer,dendrimer, and polymer) can be used. For example, the following highmolecular compound can be used: poly(N-vinylcarbazole) (abbreviation:PVK); poly(4-vinyltriphenylamine) (abbreviation: PVTPA);poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA); andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD). In addition, high molecular compounds doped with acid such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),and polyanline/poly(styrenesulfonic acid) (PAni/PSS) can be used.Further, the first layer 211 may be formed using a composite materialthat is formed using the above high molecular compound such as PVK,PVTPA, PTPDMA, or Poly-TPD and the above acceptor substance.

The second layer 212 is a layer including a substance having a high holetransporting property. As a low molecular organic compound of asubstance having a high hole transporting property, aromatic aminecompounds such as NPB (or α-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation; DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB) can be used. These substances are mainly substanceseach having a hole mobility of 10⁻⁶ cm²/Vs or higher.

However, other substances than these may also be used as long as thehole transporting properties thereof are higher than the electrontransporting properties. The layer including a substance having a highhole transporting property is not limited to a single layer, but two ormore layers including the aforementioned substances may be stacked.Further, as the second layer 212, high molecular compounds such as PVK,PVTPA, PTPDMA, and Poly-TPD can be used.

The third layer 213 is a layer including a substance having a highlight-emitting property, which corresponds to a light-emitting layer ofthe present invention, and various materials can be used. Specifically,as a low molecular organic compound of a light-emitting material thatexhibits emission of bluish light,N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstylbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like can be given. As a light-emittingmaterial that exhibits emission of greenish light,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazole-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracene-2-amine (abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like can be given. As a light-emittingmaterial that exhibits emission of yellowish light, rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),and the like can be given. As a light-emitting material that exhibitsemission of reddish light,N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like can be given.

Since the layer for controlling movement of carriers is provided betweenthe light-emitting layer and the second electrode serving as a cathodein this embodiment mode, the light-emitting layer preferably has anelectron transporting property. In a case where the light-emitting layerhas an electron transporting property, an electron blocking layer isconventionally provided between the light-emitting layer and an anode inorder to prevent penetration of electrons from the light-emitting layer.However, when the electron blocking function deteriorates with time, arecombination region expands to the inside of the electron blockinglayer (or the inside of the hole transporting layer), and reduction incurrent efficiency (that is, luminance decay) becomes significant. Onthe other hand, in the present invention, the electron blocking layer isprovided before the light-emitting layer (on the cathode side) tocontrol movement of electrons. Therefore, even when balance of theelectrons (e.g., mobility or the amount of elections relative to that ofholes) is more or less lost, there is an advantage that the ratio ofrecombination in the light-emitting layer is hardly changed andluminance is hardly lowered.

It is to be noted that the light-emitting layer may have a structure inwhich the above substance having a high light-emitting property isdispersed in another substance. As a material in which the substancehaving light-emitting property is dispersed, various kinds of materialscan be used, and a substance is preferably used, which has a lowestunoccupied molecular orbital level (LUMO level) that is higher than thatof the substance having light-emitting property and has a highestoccupied molecular orbital level (HOMO level) that is lower than that ofthe substance having light-emitting property.

Since, the layer for controlling movement of carriers is providedbetween the light-emitting layer and the second electrode serving as acathode in this embodiment mode, the light-emitting layer preferably hasan electron transporting property. That is, it is preferable that theelectron transporting property be higher than the hole transportingproperty. Therefore, as a material in which a substance having a highlight-emitting property is dispersed, an organic compound having anelectron transporting property is preferably used. Specifically, thefollowing materials can be used: metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), andbathocuproine (abbreviation: BCP); or condensed aromatics such as9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDBA),9,9′-bianthryl (BANT), 9,9′-(stilben-3,3′-diyl)diphenanthrene(abbreviation: DPNS), 9,9′-(stilben-4,4′-diyl)diphenanthrene(abbreviation: DPNS2), and 3,3′,3″-(benzene-1,3,5-triyl)tripyrene(abbreviation: TPB3).

Further, as the material in which a substance having light-emittingproperty is dispersed, a plural kinds of materials can be used. Forexample, in order to suppress crystallization, a substance forsuppressing crystallization of rubrene or the like may be further added.Furthermore, in order to efficiently transfer energy to the substancehaving light-emitting property, NPB, Alq or the like may be furtheradded. When the substance having a high light-emitting property isdispersed in another substance, crystallization of the third layer 213can be suppressed. Further, concentration quenching due to highconcentration of the substance having a high light-emitting property canbe suppressed.

As the third layer 213, high molecular compounds can be used.Specifically, as a light-emitting material that exhibits emission ofbluish light, poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,5-dimethoxybenzen-1,4-diyl)](abbreviation: PF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation: TAB-PFH), or the like can be given. As a light-emittingmaterial that exhibits emission of greenish light,poly(p-phenylenvinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazol-4,7-diyl)](abbreviation: PFBT),poly[(9,9-dioctyl-2,7-divinylenfluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],or the like can be given. As a light-emitting material that exhibitsemission of orangish to reddish light,poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene] (abbreviation:MEH-PPV), poly(3-butylthiophene-2,5-diyl),poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD), or the like can be given.

The fourth layer 214 is a layer for controlling movement of carriers.The fourth layer 214 includes two or more kinds of substances. In thefourth layer 214, the proportion of weight of the first organic compoundis greater than the proportion of weight of the second organic compound,and the first organic compound and the second compound have transportcarriers of different polarity from each other. In this embodiment mode,a case where the layer for controlling movement of carriers is providedbetween the third layer 213, which has a light-emitting function(light-emitting layer) and the second electrode 204, which serves as acathode, is explained.

In a case where the layer for controlling movement of carriers isprovided closer to the side of the second electrode serving as a cathodethan to the light-emitting layer, the first organic compound ispreferably an organic compound having electron transporting property,and the second organic compound is preferably an organic compound havinghole transporting property. That is, the first organic compound ispreferably a substance in which an electron transporting property ishigher than a hole transporting property. The second organic compound isa preferably a substance in which a hole transporting property is higherthan an electron transporting property. Further, the difference betweenthe lowest unoccupied molecular orbital level (LUMO level) of the firstorganic compound and the lowest unoccupied molecular orbital level (LUMOlevel) of the second organic compound is preferably less than 0.3 eV,further preferably, 0.2 eV or less. That is, thermodynamically, it ispreferable that electrons that are carriers be easily moved between thefirst organic compound and the second organic compound.

FIG. 4 shows a conceptual view of a layer for controlling movement ofcarriers shown in this embodiment mode. In FIG. 4, since a first organiccompound 221 has electron transporting property, electrons are easilyinjected thereto and moved to near another first organic compound. Thatis, the rate at which electrons are injected to the first organiccompound and the rate at which the electrons are discharged from thefirst organic compound are high.

On the other hand, since a second organic compound 222, which is anorganic compound having hole transporting property, has a LUMO levelthat is close to a LUMO level of the first organic compound, electronscan be injected to the first organic compound 221, thermodynamically.However, the rate (v₁) at which electrons are injected from the firstorganic compound 221, which is an organic compound having electrontransporting property, to the second organic compound 222, which is anorganic compound having hole transporting property, or the rate (v₂), atwhich electrons are injected from the second organic compound 222 to thefirst organic compound 221, is lower than the rate (v) at whichelectrons are injected from the first organic compound 221 to anotherfirst organic compound 221.

Therefore, by inclusion of the second organic compound, the electrontransporting rate of the entire layer becomes lower than that of thelayer including only the first organic compound. That is, by addition ofthe second organic compound, movement of carriers can be controlled.Further, by control of the concentration of the second organic compound,the rate of movement of carriers can be controlled.

For example, in a conventional light-emitting element in which thefourth layer 214 that is a layer for controlling movement of carriers isnot provided, electrons are injected to the third layer 213 in a statewhere the rate of movement of electrons is not lowered, and theelectrons reach the vicinity of the interface between the third layer213 and the second layer 212. Therefore, a light-emitting region isformed in the vicinity of the interface between the second layer 212 andthe third layer 213. In that case, the electrons reach the second layer212, and therefore, there is a possibility in that the second layer 212is deteriorated. As the number of electrons reaching the second layer212 is increased with time, probability of recombination of carriers isreduced with time; accordingly, reduction in element lifetime (luminancedecay with time) is realized.

In the light-emitting element of the present invention, electronsinjected from the second electrode 204 pass through the sixth layer 216including a substance having a high electron injecting property and thefifth layer 215 including a substance having a high electrontransporting property, and are injected to the fourth layer 214, whichis a layer for controlling movement of carriers. Movement rate of theelectrons injected to the fourth layer 214 becomes slow, and theelectron injection to the third layer 213 is controlled. As a result,the light-emitting region is formed in a region from the third layer 213to the vicinity of the interface between the third layer 213 and thefourth layer 214 in the light-emitting element of the present invention,although the light-emitting region has been formed in the vicinity ofthe interface between the second layer 212 including a substance havinga high hole transporting property and the third layer 213 in aconventional light-emitting element. Accordingly, the possibility thatthe electrons reach the second layer 212, which could deteriorate thesecond layer 212 including a substance having a high hole transportingproperty, is reduced. Further, regarding holes, since the fourth layer214 includes the first organic compound having an electron transportingproperty, there is a low possibility in that the fifth layer 215including a substance having a high electron transporting property isdeteriorated by holes reaching the fifth layer 215.

Furthermore, it is important in the present invention that an organiccompound having a hole transporting property is added to an organiccompound having an electron transporting property in the fourth layer214, instead that a substance electron mobility of which is low is justapplied. When such a structure is employed, in addition to just controlof electron injection to the third layer 213, which is a light-emittinglayer, change with time of the number of electron injection that iscontrolled can be suppressed. Accordingly, in the light-emitting elementof the present invention, a phenomenon can be prevented, in whichcarrier balance is deteriorated with time and probability ofrecombination is reduced; therefore, improvement in element lifetime(suppression of luminance decay with time) is achieved.

As described above, in the light-emitting element of the presentinvention, since a light-emitting region is hard to be formed at theinterface between the light-emitting layer and the hole transportinglayer or at the interface between the light-emitting layer and theelectron transporting layer, the light-emitting element is hard todeteriorate due to proximity of the light-emitting region to the holetransporting layer or the electron transporting layer. In addition,change in carrier balance with time (particularly, change with time ofthe quantity of electron injection) can be suppressed. Therefore, alight-emitting element having little deterioration and a long lifetimecan be obtained.

As described above, the first organic compound is preferably an organiccompound having an electron transporting property in this embodimentmode. Specifically, metal complexes such as Alq, Almq₃, BeBq₂, BAlq,Znq, ZnPBO, and ZnBTZ; heterocyclic compounds such as PBD, OXD-7, TAZ,TPBI, BPhen, and BCP; or condensed aromatics such as CzPA, DPCzPA, DPPA,DNA, t-BuDNA, BANT, DPNS, DPNS2, and TPB3 can be used. Further, highmolecular compounds can be used, such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy).

As the second organic compound, an organic compound having a holetransporting property is preferably used. Specifically, the followingcan be used: condensed aromatic hydrocarbons such as9,10-diphenylanthracene (abbreviation: DPAnth) and6,12-dimethoxy-5,11-diphenycrysene; aromatic amine compounds such asN,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, and BSPB; orcompounds including an amino group such as Coumarin 7 or Coumarin 30.Furtherer, high molecular compounds such as PVK, PVTPA, PTPDMA, andPoly-TPD can be used.

By the above combination, movement of electrons from the first organiccompound to the second organic compound or movement of electrons fromthe second organic compound to the first organic compound is suppressed,whereby the movement rate of electrons in the layer for controllingmovement of carriers can be suppressed. The layer for controllingmovement of carriers is formed, by dispersing the second organiccompound in the first organic compound; therefore, crystallization oragglomeration is hardly caused with time. Accordingly, theabove-described suppression effect of movement of electrons is hardlychanged with time, and as a result, the carrier balance is hardlychanged with time. This leads to improvement in lifetime of thelight-emitting element, in other words, improvement in reliability.

It is to be noted that, in the above combination, a metal complex as thefirst organic compound and an aromatic amine compound as the secondorganic compound are preferably combined. A metal complex has a highelectron transporting property and a large magnitude of its dipolemoment, whereas an aromatic amine compound has a high hole transportingproperty and a comparatively small magnitude of its dipole moment. Insuch a manner, by combination of substances dipole moments of which arelargely different from each other, the above suppression effect ofmovement of electrons becomes further significant. Specifically, whenthe magnitude of the dipole moment of the first organic compound is P₁and the magnitude of the dipole moment of the second organic compound isP₂, a combination of P₁/P₂≧3 or P₁/P₂≦0.33 is preferably satisfied. Forexample, dipole moment of Alq that is a metal complex is 9.40 debye, andthe dipole moment of 2PCAPA that is an aromatic amine compound is 1.15debye. Accordingly, when an organic compound having electrontransporting property like a metal complex is used as the first organiccompound and an organic compound having hole transporting property likean aromatic amine compound is used as the second organic compound,P₁/P₂≧3 is preferably satisfied.

The emission color of the second organic compound included in the fourthlayer 214 and the emission color of the substance having a highlight-emitting property included in the third layer 213 are preferablysimilar colors. Specifically, the difference between the wavelength ofthe highest peak of the emission spectrum of the second organic compoundand the wavelength of the highest peak of the emission spectrum of thesubstance having light-emitting property is preferably within 30 nm.When the difference is within 30 nm, the emission color of the secondorganic compound and the emission color of the substance having a highlight-emitting property can be similar colors. Accordingly, even in acase where the second organic compound emits light due to change ofvoltage or the like, change in emission color can be suppressed.However, the second organic compound has no necessity to emit light.

Further, the thickness of the fourth layer 214 is preferably greaterthan or equal to 5 nm and less than or equal to 20 nm. When the fourthlayer 214 is too thick, the rate of movement of carries is excessivelyreduced, and a driving voltage is increased. In addition, the emissionintensity of the fourth layer may be increased. Alternatively, when thefourth layer is too thin, the function of controlling movement ofcarriers is not realized. Therefore, the thickness is preferably greaterthan or equal to 5 nm and less than or equal to 20 nm.

The fifth layer 215 is a layer including a substance having a highelectron transporting property. For example, as a low molecular organiccompound, metal complexes such as Alq, Almq₃, BeBq₂, BAlq, Znq, ZnPBO,and ZnBTZ, or the like can be used. Besides the metal complex,heterocyclic compounds such as PBD, OXD-7, TAZ, TPBI, BPhen, and BCP canbe used. The substances mentioned here are mainly substances having anelectron mobility of 10⁻⁶ cm²/Vs or higher. If a substance has a higherelectron transporting property than a hole transporting property,substances other than the above may be used for the electrontransporting layer. Further, the electron transporting layer may be astacked layer of two or more layers formed from the above substances aswell as a single layer.

As the fifth layer 215, high molecular compounds may be used. Forexample, poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy), and the like can be used.

The sixth layer 216 is a layer including a substance having a highelectron injecting property. As the substance having a high electroninjecting property, alkali metals, alkaline earth metals, or compoundsthereof such as lithium fluoride (LiF), cesium fluoride (CsF), andcalcium fluoride (CaF₂) can be used. Further, a layer of a substancehaving electron transporting property, which contains alkali metals,alkaline earth metals, or compounds thereof such as Alq containingmagnesium (Mg), may be used. With the use of a layer of a substancehaving electron transporting property that contains an alkali metal oran alkaline earth metal as the electron injecting layer, electroninjection from the second electrode 204 is performed efficiently, whichis preferable.

As a substance for forming the second electrode 204, a metal, an alloy,an electroconductive compound, or a mixture thereof, or the like with alow work function (specifically, a work function of 3.8 eV or lower) canbe used. As a specific example of such a cathode material, an elementthat belongs to Group 1 or 2 of the periodic table, that is, alkalimetals such as lithium (Li) and cesium (Cs), alkaline earth metals suchas magnesium (Mg), calcium (Ca), and strontium (Sr), alloys containingthese (MgAg, AlLi, or the like), rare earth metals such as europium (Eu)and ytterbium (Yb), alloys containing these, and the like can be given.A film of an alkali metal, an alkaline earth metal, an alloy containingthese can be formed by a sputtering method. Further, an alloy containingan alkali metal or an alkaline earth metal can be formed by a sputteringmethod. A silver past or the like can be formed by an inkjet method orthe like.

Further, by the sixth layer 216, which has a function of promotingelectron injection being provided between the second electrode 204 andthe fifth layer 215, various conductive materials such as Al, Ag, ITO,or indium oxide-tin oxide containing silicon or silicon oxide can beused as the second electrode 204 regardless of their work functions.These conductive materials can be formed by sputtering, inkjet, spincoating or the like.

Various methods can be used for forming the EL layer, regardless of adry method or a wet method. For example, a vacuum evaporation, inkjet,spin coating or the like may be used. Furthermore, the electrodes or thelayers may be formed by different film formation methods. For example,the EL layer may be formed by a wet method using a high molecularcompound selected from the above-described materials. Further, the ELlayer can also be formed by a wet method using a low molecular organiccompound. Furthermore, the EL layer may be formed by a dry method suchas vacuum evaporation using a low molecular organic compound.

The electrode may be formed by a wet method using sol-gel method, or bya wet method using a paste of a metal material. Further, the electrodemay be formed by a dry method such as sputtering or vacuum evaporation.

Hereinafter, specific methods of forming a light-emitting element aredescribed. In a case where the light-emitting element of the presentinvention is applied to a display device and light-emitting layers areformed by separately applying mixture including a material for thelight-emitting layers, the light-emitting layers are preferably formedby a wet method. When the light-emitting layers are formed using inkjet,it becomes easy to form the light-emitting layers by separately applyingmixture including a material for the light-emitting layers even on alarge-sized substrate.

For example, in the structure shown in FIGS. 1A and 1B, the firstelectrode may be formed by sputtering, which is a dry method, the firstlayer may be formed by inkjet or spin coating, which is a wet method,the second layer may be formed by vacuum evaporation, which is a drymethod, the third layer may be formed by inkjet, which is a wet method,the fourth layer may be formed by co-evaporation, which is a dry method,the fifth layer and the sixth layer may be formed by vacuum evaporation,which is a dry method, and the second electrode may be formed by inkjetor spin coating, which is a wet method.

Alternatively, the first electrode may be formed by inkjet, which is awet method, the first layer may be formed by vacuum evaporation, whichis a dry method, the second layer may be formed by inkjet or spincoating, which is a wet method, the third layer may be formed by inkjet,which is a wet method, the fourth layer may be formed by inkjet or spincoating, which is a wet method, the fifth layer and the sixth layer maybe formed by inkjet or spin coating, which is a wet method, and thesecond electrode may be formed by inkjet or spin coating, which is a wetmethod. The method for forming the light-emitting element is notparticularly limited to the above method, and a wet method and a drymethod may be combined as appropriate.

For example, in the case of the structures shown in FIGS. 1A and 1B, thefirst electrode can be formed by sputtering, which is a dry method, thefirst layer and the second layer can be formed by inkjet or spincoating, which is a wet method, the third layer, which is alight-emitting layer, can be formed by inkjet, which is a wet method,the fourth layer can be formed by co-evaporation, which is a dry method,the fifth layer and the sixth layer can be formed by vacuum evaporation,which is a dry method, and the second electrode can be formed by vacuumevaporation, which is a dry method. That is, over a substrate over whichthe first electrode is formed in the predetermined shape, the firstlayer to third layer can be formed by wet methods, and the fourth layerto the second electrode can be formed by dry methods. By this method,the first layer to the third layer can be formed under an atmosphericpressure, and it is easy to form the third layers by separately applyingmixture including a material for the light-emitting layers. Further, thefourth layer to the second electrode can be consecutively formed in avacuum. Therefore, the steps can be simplified, and productivity can beimproved.

One example is shown below. PEDOT/PSS is faulted as the first layer overthe first electrode. Since PEDOT/PSS has water solubility, PEDOT/PSS canbe formed as an aqueous solution by a spin coating method, an inkjetmethod, or the like. The third layer is provided as a light-emittinglayer over the first layer without providing the second layer. Thelight-emitting layer can be formed by an inkjet method using a solutionin which a substance having light-emitting property is dissolved in asolvent (such as toluene, dodecylbenzene, or a mixed solvent ofdodecylbenzene and tetralin) that does not dissolve the first layer(PEDOT/PSS) that is previously formed. Next, the fourth layer is formedover the third layer. In the case of forming the fourth layer by a wetmethod, the fourth layer is required to be formed using a solvent thatdoes not dissolves the first layer that is previously formed and thirdlayer. In that case, since the selection range of the solvent isnarrowed, the fourth layer is easily formed using a dry method.Therefore, when the fourth layer to the second electrode areconsecutively formed in a vacuum by a vacuum evaporation method that isa dry method, the steps can be simplified.

In the case of the structure of FIGS. 2A and 2B, the light-emittingelement can be formed in the reverse order of the above method asfollows: the second electrode can be formed by a sputtering method or avacuum evaporation method that is a dry method; the sixth layer and thefifth layer can be formed by a vacuum evaporation method that is a drymethod; the fourth layer can be formed by a co-evaporation method thatis a dry method; the third layer can be formed by an inkjet method thatis a wet method; the second layer and the first layer can be formed byan inkjet method and a spin coating method that is a wet method; and thefirst electrode can be formed by an inkjet method or a spin coatingmethod that is a wet method. By this method, the second electrode to thefourth layer can be consecutively formed in a vacuum by a dry method,and the third layer to the first electrode can be formed in anatmospheric pressure. Therefore, the steps can be simplified, andproductivity can be improved.

In the light-emitting element having the above structure of the presentinvention, current flows by a potential difference generated between thefirst electrode 202 and the second electrode 204, and holes andelectrons are recombined in the EL layer 203, whereby the light-emittingelement emits light. Light emission is extracted to the outside throughone of or both the first electrode 202 and the second electrode 204.Accordingly, one of or both the first electrode 202 and the secondelectrode 204 are an electrode having light transmitting property.

When only the first electrode 202 has light transmitting property, lightemission is extracted from the substrate side through the firstelectrode 202 as shown in FIG. 3A. When only the second electrode 204has light transmitting property, light emission is extracted from theopposite side of the substrate through the second electrode 204 as shownin FIG. 3B. When both the first electrode 202 and the second electrode204 have light transmitting property, light emission is extracted fromboth the substrate side and the opposite side to the substrate throughthe first electrode 202 and the second electrode 204 as shown in FIG.3C.

The structure of the layer provided between the first electrode 202 andthe second electrode 204 is not limited to the above structure. Anystructure other than the above may be employed as long as alight-emitting region in which holes and electrons are recombined isprovided in a portion away from the first electrode 202 and the secondelectrode 204 in order to prevent quenching caused by proximity of thelight-emitting region to a metal, and a layer for controlling movementof carriers is provided.

That is, the stacked structure of the EL layer is not particularlylimited. The EL layer may be formed by appropriate combination of alayer including a substance having a high electron transportingproperty, a substance having a high hole transporting property, asubstance having a high electron injecting property, a substance havinga high hole injecting property, a substance having a bipolar property (asubstance having a high electron and hole transporting properties), andthe like; a layer for controlling movement of carriers shown in thisembodiment mode; and a light-emitting layer.

Since a layer for controlling movement of carriers shown in thisembodiment mode is a layer for controlling movement of electrons, thelayer for controlling movement of carriers is preferably providedbetween the electrode serving as a cathode and the light-emitting layer.For example, as shown in FIG. 1B, a seventh layer 217 including asubstance having a high electron transporting property may be providedbetween the third layer 213 having a light-emitting function and thefourth layer 214 that is a layer for controlling movement of carriers.

Further preferably, the layer for controlling movement of carriers isprovided so as to be in contact with the light-emitting layer. When thelayer for controlling movement of carriers is provided so as to be incontact with the light-emitting layer, electron injection to thelight-emitting layer can be directly controlled. Therefore, change incarrier balance with time can be further suppressed in thelight-emitting element, and further increased effect of improvement inelement lifetime can be obtained. In addition, the seventh layer 217including a substance having a high electron transporting property doesnot need to be provided, whereby the process becomes simple.

It is to be noted that the layer for controlling movement of carriers ispreferably provided so as to be in contact with the light-emittinglayer, and in such a case, the structures of the first organic compoundincluded in the layer for controlling movement of carriers and anorganic compound occupying large part of the light-emitting layer arepreferably different from each other. In particular, in the case wherethe light-emitting layer includes a substance in which a substancehaving a high light-emitting property (a third organic compound) isdispersed and the substance having a high light-emitting property (afourth organic compound), the structures of the third organic compoundand the first organic compound are preferably different from each other.In such a structure, movement of carriers (movement of electrons in thisembodiment mode) from the layer for controlling movement of carriers tothe light-emitting layer is suppressed also between the first organiccompound and the third organic compound, and effect of providing thelayer for controlling movement of carriers is further increased.

Further, the light-emitting element shown in FIG. 2A has a structure inwhich the second electrode 204 serving as a cathode, the EL layer 203,and the first electrode 202 serving as an anode are sequentially stackedover the substrate 201. The EL layer 203 has a first layer 211, a secondlayer 212, a third layer 213, a fourth layer 214, a fifth layer 215, anda sixth layer 216. The fourth layer 214 is provided between the secondelectrode serving as the cathode and the third layer.

In this embodiment mode, the light-emitting element is manufactured overa substrate made of glass, plastic, or the like. When a plurality ofsuch light-emitting elements are manufactured over one substrate, apassive matrix light-emitting device can be manufactured. Moreover, forexample, a thin film transistor (Iii) may be formed over a substratemade of glass, plastic, or the like so that a light-emitting element ismanufactured to be electrically connected to the TFT. Thus, an activematrix light-emitting device in which driving of the light-emittingelement is controlled by the TFT can be manufactured.

The structure of the TFT is not particularly limited. The TFT may beeither a staggered type or an inversely staggered type. In addition, adriver circuit formed over a TFT substrate may be formed using bothN-channel TFTs and P-channel TFTs, or using either N-channel TFTs orP-channel TFT. The crystallinity of a semiconductor film used for theTFT is not particularly limited, either. An amorphous semiconductor filmmay be used, or a crystalline semiconductor film may be used.

The light-emitting element of the present invention has a layer forcontrolling movement of carriers. Since the layer for controllingmovement of carriers includes two or more kinds of substances, carrierbalance can be controlled precisely by control of combination, themixture ratio, the thickness, or the like of the substances. Since thecarrier balance can be controlled by control of combination, the mixtureratio, the thickness, or the like of the substances, control of carrierbalance can be easier than the conventional technique. That is, even ifa physical property of the material itself is not changed, the movementof carriers can be controlled by control of the mixture ratio, thethickness, or the like.

When the carrier balance is controlled using the layer for controllingmovement of carriers, the light emission efficiency of thelight-emitting element can be improved. With the use of the layer forcontrolling movement of carriers, excessive injection of electrons intothe light-emitting layer and reach of electrons that penetrate thelight-emitting layer to a hole transporting layer or a hole injectinglayer can be suppressed. When electrons reach the hole transportinglayer or the hole injecting layer, probability of recombination ofcarriers in the light-emitting layer is reduced (in other words, thecarrier balance grow undesirable), which leads to reduction in lightemission efficiency with time. That is, lifetime of the light-emittingelement becomes short.

However, as shown in this embodiment mode, when the layer forcontrolling movement of carriers is used, excessive injection ofelectrons into the light-emitting layer and reach of electrons thatpenetrate the light-emitting layer to the hole transporting layer or thehole injecting layer can be suppressed, and reduction in light emissionefficiency with time can be suppressed. That is, a light-emittingelement with a long lifetime can be obtained in the present invention.More specifically, movement of carriers is controlled using the secondorganic compound the proportion of weight of which is lower than theproportion of weight of the first organic compound in the two or moresubstances included in the layer for controlling movement of carriers.Therefore, the movement of carriers can be controlled by a component theproportion of weight of which is lower in components included in thelayer for controlling movement of carriers. As a result, alight-emitting element that hardly deteriorates with time and has a longlifetime can be realized.

That is, change in carrier balance is hardly caused in thelight-emitting element as compared with the case where carrier balanceis controlled by a single substance. For example, in a case where themovement of carriers is controlled by a layer formed from a singlesubstrate, the carrier balance of the entire layer is changed due topartial change in morphology or partial crystallization. Therefore, thelayer for controlling movement of carriers in that case easilydeteriorates with time. However, as shown in this embodiment mode, themovement of carriers is controlled by a component the proportion ofweight of which is lower in the components included in the layer forcontrolling movement of carriers, whereby change in morphology or effectof crystallization, aggregation, or the like becomes reduced, and thusdeterioration with time is hardly caused. Thus, a light-emitting elementwith a long lifetime can be obtained in which reduction in lightemission efficiency with time is hardly caused.

As shown in this embodiment mode, a structure in which the layer forcontrolling movement of carriers is provided between the light-emittinglayer and the second electrode serving as a cathode is particularlyeffective when the structure is applied to the light-emitting layerhaving excessive electrons. For example, in a case where thelight-emitting layer has an electron transporting property and there isa possibility in that the ratio in which electrons injected from thesecond electrode penetrate the light-emitting layer is increased withtime, it is particularly effective to apply the structure shown in thisembodiment mode.

This embodiment mode can be combined with another embodiment mode asappropriate.

Embodiment Mode 2

In this embodiment mode, a mode of a light-emitting element, which isdifferent from that of Embodiment Mode 1, of the present invention willbe explained with reference to FIG. 5A. In this embodiment mode, alight-emitting element provided with a layer for controlling movement ofholes as a layer for controlling movement of carriers will be explained.The light-emitting element of the present invention has a plurality oflayers between a pair of electrodes. The plurality of layers are stackedby combining of layers including a substance having a high carrierinjecting property and a substance having a high carrier transportingproperty so that a light-emitting region is formed in a portion awayfrom the electrodes, in other words, carriers are recombined in aportion away from the electrodes.

In this embodiment mode, the light-emitting element includes a firstelectrode 402, a second electrode 404, and an EL layer 403 providedbetween the first electrode 402 and the second electrode 404. Thisembodiment mode is explained on the assumption that the first electrode402 serves as an anode and the second electrode 404 serves as a cathode.That is, this embodiment mode is explained below on the assumption thatlight emission is obtained when voltage is applied between the firstelectrode 402 and the second electrode 404 so that the potential of thefirst electrode 402 is higher than that of the second electrode 404. Asa substrate 401, the similar substrate as that of Embodiment Mode 1 canbe used.

As the first electrode 402, a metal, an alloy, an electroconductivecompound, a mixture thereof, or the like with a high work function(specifically, a work function of 4.0 eV or higher) is preferably used,and the similar material as those shown in Embodiment Mode 1 can beused.

The EL layer 403 has a first layer 411, a second layer 412, a thirdlayer 413, a fourth layer 414, a fifth layer 415, and a sixth layer 416.It is to be noted that any stacked structure is acceptable as long asthe EL layer 403 has a layer for controlling movement of carriers shownin this embodiment mode and a light-emitting layer, similarly toEmbodiment Mode 1, and a stacked structure of layers other than theabove layers is not particularly limited. For example, a hole injectinglayer, a hole transporting layer, a light-emitting layer, a layer forcontrolling movement of carriers, an electron transporting layer, anelectron injecting layer, and the like can be combined as appropriate.

The first layer 411 is a layer including a substance having a high holeinjecting property, and the similar substance as those shown inEmbodiment Mode 1 can be used.

The second layer 412 is a layer including a substance having a high holetransporting property, and the similar substance as those shown inEmbodiment Mode 1 can be used.

The third layer 413 is a layer for controlling movement of carriers. Thethird layer 413 includes two or more kinds of substances. In the thirdlayer 413, the proportion of weight of a first organic compound isgreater than the proportion of weight of a second organic compound, andthe first organic compound and the second organic compound havetransport carriers of different polarity from each other. In thisembodiment mode, a case is explained, where the layer for controllingmovement of carriers is provided between the first electrode serving asan anode and the light-emitting layer. That is, a case is explained,where the layer for controlling movement of carriers is provided betweenthe fourth layer 414 having a light-emitting function and the firstelectrode 402.

In the case where the layer for controlling movement of carriers isprovided between the first electrode serving as an anode and thelight-emitting layer, the first organic compound is preferably anorganic compound having hole transporting property, and the secondorganic compound is preferably an organic compound having electrontransporting property. That is, the first organic compound is preferablya substance in which a hole transporting property is higher than anelectron transporting property. The second organic compound ispreferably a substance in which an electron transporting property ishigher than a hole transporting property. Further, the differencebetween the highest occupied molecular orbital (HOMO level) of the firstorganic compound and the highest occupied molecular orbital (HOMO level)of the second organic compound is preferably less than 0.3 eV, furtherpreferably, 0.2 eV or less. That is, thermodynamically, it is preferablethat holes that are carriers be easily moved between the first organiccompound and the second organic compound.

FIG. 8 shows a conceptual view of a layer for controlling movement ofcarriers shown in this embodiment mode. In FIG. 8, since a first organiccompound 422 has a hole transporting property, holes are easily injectedthereto and moved to near another first organic compound. That is, therate at which holes are injected to the first organic compound and therate (v) at which holes are discharged from the first organic compoundare large.

On the other hand, since a second organic compound 421, which is anorganic compound having electron transporting property, has a HOMO levelthat is close to the HOMO level of the first organic compound, holes canbe thermodynamically injected. However, the rate (v₁) at which holes areinjected from the first organic compound 422, which is an organiccompound having a hole transporting property, to the second organiccompound 421, which is an organic compound having an electrontransporting property or the rate (v₂) at which holes are injected fromthe second organic compound 421 to the first organic compound 422 islower than the rate (v) at which holes are injected from the firstorganic compound 422 to another first organic compound 422.

Therefore, by the inclusion of the second organic compound, a holetransporting rate of the entire layer becomes lower than that of thelayer including only the first organic compound 422. That is, byaddition of the second organic compound, movement of carriers can becontrolled. Further, by control of concentration of the second organiccompound, a moving rate of carriers can be controlled.

For example, in a conventional light-emitting element, in which thethird layer 413 is not provided, holes are injected to the fourth layer414 in the state where the rate of holes are not reduced, and the holesreach the vicinity of the interface of the fifth layer 415. Therefore, alight-emitting region is formed in the vicinity of the interface betweenthe fourth layer 414 and the fifth layer 415. In that case, the holesreach the fifth layer 415, and there is a possibility in that the fifthlayer 415 is deteriorated. As the quantity of holes reaching the fifthlayer 415 is increased with time, probability of recombination isreduced with time; accordingly, reduction in element lifetime (luminancedecay with time) is realized.

In the light-emitting element of the present invention, holes injectedfrom the first electrode 402 are injected to the third layer 413, whichis a layer for controlling movement of carriers, through the first layer411 including a substance having a high hole injecting property and thesecond layer 412 including a substance having a high hole transportingproperty. Rate of movement of the holes injected to the third layer 413becomes reduced, and hole injection to the fourth layer 414 iscontrolled. As a result, in the light-emitting element of the presentinvention, a light-emitting region is formed in the fourth layer 414 andthe vicinity of the interface between the fourth layer 414 and the thirdlayer 413, the element region being formed in the vicinity of theinterface between the fifth layer 415 including a substance having ahigh electron transporting property and the fourth layer 414 in theconventional light-emitting element.

Accordingly, in the light-emitting element of the present invention, thepossibility in that the fifth layer 415 including a substance having ahigh electron transporting property is deteriorated since holes reachthe fifth layer 415 is reduced. Further, regarding electrons, since thethird layer 413 includes the first organic compound having a holetransporting property, the possibility in that the electrons reach thesecond layer 412 and the second layer 412 including a substance having ahigh hole transporting property is deteriorated is reduced.

Furthermore, it is important in the present invention that a substanceof which hole mobility is low is not just applied, but an organiccompound having electron transporting property is added to the organiccompound having hole transporting property in the third layer 413. Whensuch a structure is employed, in addition to just controlling holeinjection to the fourth layer 414, change of the quantity of controlledhole injection with time can be suppressed. According to the above, inthe light-emitting element of the present invention, a phenomenon inwhich carrier balance is deteriorated with time and probability ofrecombination is reduced can be prevented; therefore, improvement inelement lifetime (suppression of luminance decay with time) is realized.

In the light-emitting element of the present invention, since alight-emitting region is not formed at an interface between thelight-emitting layer and the hole transporting layer or at an interfacebetween the light-emitting layer and the electron transporting layer,the light-emitting element is not affected by deterioration due toproximity of the light-emitting region to the hole transporting layer orthe electron transporting layer. In addition, change in carrier balancewith time (particularly, change of the quantity of electron injectionwith time) can be suppressed. Accordingly, a light-emitting elementhaving little deterioration and a long lifetime can be obtained.

As described above, the first organic compound is preferably an organiccompound having a hole transporting property in this embodiment mode.Specifically, the following can be used: condensed aromatic hydrocarbonssuch as DPAnth and 6,12-dimethoxy-5,11-diphenycrysene; or aromatic aminecompounds such as CzA1PA, DPhPA, PCAPA, PCAPBA, 2PCAPA, NPB (or α-NPD),DFLDPBi, and BSPB. Furtherer, high molecular compounds such as PVK,PVTPA, PTPDMA, and Poly-TPD can be used.

The second organic compound is preferably an organic compound having anelectron transporting property. Specifically, the following can be used:metal complexes such as Alq, Almq₃, BeBq₂, Znq, ZnPBO, and ZnBTZ;heterocyclic compounds such as PBD, OXD-7, TAZ, BPhen, and BCP; orcondensed aromatics compounds such as CzPA, DPCzPA, DPPA, DNA, t-BuDNA,BANT, DPNS, DPNS2, and TPB3. In addition, high molecular compounds canbe used, such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py) andpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy).

By the above combination, movement of holes from the first organiccompound to the second organic compound or movement of holes from thesecond organic compound to the first organic compound is suppressed, andthen, the rate of movement of holes in a layer for controlling movementof carriers can be suppressed. The layer for controlling movement ofcarriers is formed, in which the second organic compound is dispersedinto the first organic compound; therefore, crystallization oraggregation is hardly caused with time. Accordingly, the above-describedsuppression effect of movement of holes is hardly changed with time, andas a result, carrier balance is hardly changed with time. This leads toimprovement in a lifetime of the light-emitting element, in other words,improvement in reliability.

It is to be noted that, in the above combination, an aromatic aminecompound as the first organic compound and a metal complex as the secondorganic compound are preferably combined. An aromatic amine compound hasa high hole transporting property and comparatively small dipole moment,whereas, a metal complex has a high electron transporting property andlarge dipole moment. In such a manner, by combination of substancesdipole moment of which is largely different from each other, the abovesuppression effect of movement of holes becomes further significant.Specifically, when the magnitude of the dipole moment of the firstorganic compound is P₁ and the magnitude of the dipole moment of thesecond organic compound is P₂, a combination of P₁/P₂≧3 or P₁/P₂≦0.33 ispreferably satisfied. For example, the dipole moment of NPB, which is anaromatic amine compound, is 0.86 debye, and the dipole moment of Alq,which is a metal complex, shows 9.40 debye. Accordingly, when an organiccompound having a hole transporting property like an aromatic aminecompound is used as the first organic compound and an organic compoundhaving an electron transporting property like a metal complex is used asthe second organic compound, P₁/P₂≦0.33 is preferably satisfied.

The emission color of the second organic compound included in the thirdlayer 413 and the emission color of a substance having a highlight-emitting property included in the fourth layer 414 are preferablysimilar colors. Specifically, the difference between the wavelength ofthe highest peak of the emission spectrum of the second organic compoundand the wavelength of the highest peak of the emission spectrum of thesubstance having a high light-emitting property is preferably within 30nm. When the difference is within 30 nm, the emission color of thesecond organic compound and the emission color of the substance havinglight-emitting property can be similar colors. Accordingly, even in acase where the second organic compound emits light due to change ofvoltage or the like, change in emission color can be suppressed.However, the second organic compound has no necessity to emit light.

Further, a thickness of the third layer 413 is preferably greater thanor equal to 5 nm and less than or equal to 20 nm. When the third layer413 is too thick, the rate of movement of carries is excessivelyreduced, and a driving voltage is increased. In addition, emissionintensity of the third layer may be increased. Alternatively, when thethird layer 413 is too thin, the function of controlling movement ofcarriers is not realized. Therefore, the thickness is preferably greaterthan or equal to 5 nm and less than or equal to 20 nm.

The fourth layer 414 is a layer including a substance having a highlight-emitting property, in other words, the light-emitting layer, andthe substance having a high light-emitting property shown in EmbodimentMode 1 can be used for the fourth layer 414. Further, the light-emittinglayer may have a structure in which the substance having a highlight-emitting property is dispersed into another substance as shown inEmbodiment Mode 1.

In this embodiment mode, since the layer for controlling movement ofcarriers is provided between the light-emitting layer and the firstelectrode serving as an anode, the light-emitting layer has preferably ahole transporting property. That is, it is preferable that a holetransporting property be higher than an electron transporting property.In the case where the light-emitting layer has a hole transportingproperty, a hole blocking layer was conventionally provided between acathode and the light-emitting layer so as to prevent holes frompenetrating the light-emitting layer. However, when the hole blockingfunction is deteriorated with time, a recombination region is extendedto the inside of the hole blocking layer (or the inside of an electrontransporting layer), and thus reduction in current efficiency (in otherwords, luminance decay) becomes significant. On the other hand, in thepresent invention, since movement of holes is controlled between ananode and the light-emitting layer (an anode side), even if the balanceof the holes (e.g., mobility or number of electrons relative to those ofholes) is more or less lost, the ratio of carriers recombined in thelight-emitting layer is hardly changed, and there is an advantage inthat luminance is hardly lowered.

Therefore, as a material for dispersing a substance having highlight-emitting property shown in Embodiment Mode 1, an organic compoundhaving hole transporting property is preferable. Specifically, thefollowing can be used: condensed aromatic hydrocarbons such as DPAnthand 6,12-dimethoxy-5,11-diphenycrysene; or aromatic amine compounds suchas CzA1PA, DPhPA, PCAPA, PCAPBA, 2PCAPA, NPB (or α-NPD), TPD, DFLDPBi,and BSPB.

The fifth layer 415 is a layer including a substance having a highelectron transporting property, and similar substances as those shown inEmbodiment Mode 1 can be used for the fifth layer 415.

The sixth layer 416 is a layer including a substance having a highelectron injecting property, and similar substances as those shown inEmbodiment Mode 1 can be used for the sixth layer 416.

Various methods can be used for forming the EL layer, regardless of adry method or a wet method. For example, vacuum evaporation, inkjet,spin coating, or the like may be used. Furthermore, electrodes or layersmay be formed by different film formation methods. For example, the ELlayer may be formed by a wet method using a high molecular compound inthe above-described materials. Alternatively, the EL layer can be formedby a wet method using a low molecular organic compound. Furthermore, theEL layer may be formed by a dry method such as vacuum evaporation usinga low molecular organic compound.

The electrode may be formed by a wet method using a sol-gel method, orby a wet method using paste of a metal material. Further, the electrodemay be formed by a dry method such as sputtering or vacuum evaporation.

Hereinafter, specific methods of forming a light-emitting element aredescribed. In the case where the light-emitting element of the presentinvention is applied to a display device and light-emitting layers areformed by separately applying mixture including a material for thelight-emitting layers, the light-emitting layers are preferably formedby a wet method. When the light-emitting layers are formed using inkjet,it becomes easy to form light-emitting layers by separately applyingmixture including a material for the light-emitting layer seven on alarge-sized substrate.

For example, in the structures shown in FIGS. 5A and 5B, the firstelectrode may be formed by sputtering, which is a dry method, the firstlayer may be formed by inkjet or spin coating, which is a wet method,the second layer may be formed by vacuum evaporation which is a drymethod, the third layer may be formed by inkjet, which is a wet method,the fourth layer may be formed by co-evaporation, which is a dry method,the fifth layer and the sixth layer may be formed by vacuum evaporation,which is a dry method, and the second electrode may be formed by inkjetor spin coating, which is a wet method. Alternatively, the firstelectrode may be formed by inkjet, which is a wet method, the firstlayer may be formed by vacuum evaporation, which is a dry method, thesecond layer may be formed by inkjet or spin coating, which is a wetmethod, the third layer may be formed by inkjet, which is a wet method,the fourth layer may be formed by inkjet or spin coating, which is a wetmethod, the fifth layer and the sixth layer may be formed by inkjet orspin coating, which is a wet method, and the second electrode may beformed by inkjet or spin coating. The method for forming thelight-emitting element is not limited to the above methods, and a wetmethod and a dry method may be combined as appropriate.

For example, specifically, in the case of the structures shown in FIGS.5A and 5B, the first electrode can be formed by sputtering, which is adry method, the first layer and the second layer can be formed by vacuumevaporation, which is a dry method, the third layer can be formed byco-evaporation, which is a dry method, the fourth layer that is alight-emitting layer can be formed by inkjet, which is a wet method, thefifth layer can be formed by inkjet or spin coating, which is a wetmethod, and the second electrode can be formed by inkjet or spincoating, which is a wet method without providing the sixth layer. Thatis, the first electrode to the third layer can be formed by dry methods,and the fourth layer to the second electrode can be formed by wetmethods. By this method, the first electrode to the third layer can beconsecutively formed in a vacuum, and the fourth layer to the secondelectrode can be formed under an atmospheric pressure. Further, it iseasy to form the fourth layers by separately applying mixture includinga material for the fourth layers. Therefore, the steps can besimplified, and productivity can be improved.

Further, in the case of the structure shown in FIGS. 6A and 6B, alight-emitting element can be formed in a reverse order of the abovemethod as follows: the second electrode can be formed by inkjet or spincoating, which is a wet method; the sixth layer and the fifth layer canbe formed by inkjet or spin coating, which is a wet method; the fourthlayer can be formed by inkjet, which is a wet method; the third layercan be formed by co-evaporation, which is a dry method; the second layerand the first layer can be formed by vacuum evaporation, which is a drymethod; and the first electrode can be formed by vacuum evaporation,which is a dry method. By this method, the second electrode to thefourth layer can be formed in an atmospheric pressure, and the thirdlayer to the first electrode can be consecutively formed under a vacuumby a dry method. Therefore, the steps can be simplified, andproductivity can be improved.

In the light-emitting element having the above structure of the presentinvention, current flows owing to a potential difference caused betweenthe first electrode 402 and the second electrode 404, and holes andelectrons are recombined in the EL layer 403, whereby the light-emittingelement emits light. Light emission is extracted to the outside throughone of or both the first electrode 402 and the second electrode 404.Accordingly, one of or both the first electrode 402 and the secondelectrode 404 are an electrode having a transmitting property.

When only the first electrode 402 has light transmitting property, lightemission is extracted from the substrate side through the firstelectrode 402 as shown in FIG. 7A. When only the second electrode 404has light transmitting property, light emission is extracted from theopposite side of the substrate through the second electrode 404 as shownin FIG. 7B. When both the first electrode 402 and the second electrode404 have light transmitting property, light emission is extracted fromboth the substrate side and the opposite side of the substrate throughthe first electrode 402 and the second electrode 404 as shown in FIG.7C.

The structure of the layer provided between the first electrode 402 andthe second electrode 404 is not limited to the above structure. That is,in the present invention and this embodiment mode, the structure otherthan the above may be employed as long as a light-emitting region inwhich holes and electrons are recombined is provided in a portion awayfrom the first electrode 402 and the second electrode 404 in order toprevent quenching caused by proximity of the light-emitting region and ametal, and a layer for controlling movement of carriers is provided.

That is, the stacked structure of the EL layer is not particularlylimited. The EL layer may be formed by appropriate combination of layerseach including a substance having a high electron transporting property,a substance having a high hole transporting property, a substance havinga high electron injecting property, a substance having a high holeinjecting property, a substance having a bipolar property (a substancehaving a high electron and hole transporting properties), or the like; alayer for controlling movement of carriers shown in this embodimentmode; and a light-emitting layer.

It is to be noted that, since the layer for controlling movement ofcarriers shown in this embodiment mode is a layer for controllingmovement of holes, the layer for controlling movement of carriers ispreferably provided between the electrode serving as an anode and thelight-emitting layer. For example, as shown in FIG. 5B, a seventh layer417 including a substance having a high hole transporting property maybe provided between the fourth layer 414 having light-emitting functionand the third layer 413, which is a layer for controlling movement ofcarriers.

Further preferably, the layer for controlling movement of carriers ispreferably provided so as to be in contact with the light-emittinglayer. When the layer for controlling movement of carriers is providedso as to be in contact with the light-emitting layer, hole injection tothe light-emitting layer can be directly controlled. Therefore, changein carrier balance with time can be further suppressed in thelight-emitting element, and further improvement in element lifetime canbe obtained. In addition, the seventh layer 217 including a substancehaving a high hole transporting property does not need to be provided,whereby the process becomes simple.

In the case where the layer for controlling movement of carriers isprovided so as to be in contact with the light-emitting layer, thestructures of the first organic compound included in the layer forcontrolling movement of carriers and an organic compound occupying largepart of the light-emitting layer are preferably different from eachother. In particular, in the case where the light-emitting layerincludes a substance for dispersing a substance having a highlight-emitting property (a third organic compound) and a substancehaving a high light-emitting property (a fourth organic compound), thestructures of the third organic compound and the first organic compoundare preferably different from each other. In such a structure, movementof carriers (movement of holes in this embodiment mode) from the layerfor controlling movement of carriers to the light-emitting layer issuppressed also between the first organic compound and the third organiccompound, and the effect of providing the layer for controlling movementof carriers is further increased.

The light-emitting element shown in FIGS. 6A and 6B includes, over asubstrate 401, a second electrode 404 serving as a cathode, an EL layer403, and a first electrode 402 serving as an anode are sequentiallystacked. The EL layer 403 has a first layer 411, a second layer 412, athird layer 413, a fourth layer 414, a fifth layer 415, and a sixthlayer 416. The third layer 413 is provided between the first electrodeserving as an anode and the fourth layer 414.

The light-emitting element of the present invention has a layer forcontrolling movement of carriers. Since the layer for controllingmovement of carriers includes two or more kinds of substances, carrierbalance can be controlled precisely by control of combination, themixture ratio, the thickness, or the like of the substances. Since thecarrier balance can be controlled by control of combination, the mixtureratio, the thickness, or the like of the substances, control of carrierbalance can be easier than the conventional technique. That is, even ifa physical property of the material itself is not changed, the movementof carriers can be controlled by control of the mixture ratio, thethickness, or the like. The carrier balance is improved using the layerfor controlling movement of carriers, whereby light emission efficiencyof the light-emitting element can be improved.

With the use of the layer for controlling movement of carriers,excessive injection of holes and reach of holes that penetrate thelight-emitting layer to an electron transporting layer or an electroninjecting layer can be suppressed. When holes reach the electrontransporting layer or the electron injecting layer, probability ofrecombination in the light-emitting layer is reduced (in other words,the carrier balance is lost), which leads to reduction in light emissionefficiency with time. That is, lifetime of the light-emitting elementbecomes short. However, as shown in this embodiment mode, when the layerfor controlling movement of carriers is used, excessive injection ofholes and reach of holes that penetrate the light-emitting layer to theelectron transporting layer or the electron injecting layer can besuppressed, and reduction in light emission efficiency with time can besuppressed. That is, a light-emitting element with long lifetime can beobtained.

Movement of carriers is controlled using the second organic compound theproportion of weight of which is lower than the proportion of weight ofthe first organic compound in the two or more kinds of substancesincluded in the layer for controlling movement of carriers. Therefore,since movement of carriers can be controlled by a component theproportion of weight of which is lower in components included in thelayer for controlling movement of carriers, long lifetime of thelight-emitting element that hardly deteriorates with time can berealized. That is, change in carrier balance is hardly caused ascompared with the case where carrier balance is controlled by a singlesubstance.

For example, in the case where movement of carriers is controlled by alayer formed from a single substance, the balance of the entire layer ischanged due to partial change in morphology or partial crystallization.Therefore, the layer for controlling movement of carriers in that caseeasily deteriorates with time. However, as shown in this embodimentmode, the movement of carriers is controlled by a component theproportion of weight of which is lower in the components included in thelayer for controlling movement of carriers, whereby change in morphologyor effect of crystallization, aggregation, or the like is reduced, andthen change with time is hardly caused. Thus, a light-emitting elementwith a long lifetime, in which reduction in light emission efficiencywith time is hardly caused, can be obtained.

As shown in this embodiment mode, a structure in which the layer forcontrolling movement of carriers is provided between the light-emittinglayer and the first electrode serving as an anode is particularlyeffective when the structure is applied to the light-emitting layerhaving excessive holes. For example, in the case where thelight-emitting layer has a hole transporting property, and in the casewhere there is a possibility in that the ratio in which holes injectedfrom the first electrode penetrate the light-emitting layer is increasedwith time, it is particularly effective that the structure shown in thisembodiment mode is applied.

This embodiment mode can be combined with another embodiment mode asappropriate. For example, a layer for controlling movement of holes maybe provided between a light-emitting layer and a first electrode servingas an anode, and a layer for controlling movement of electrons may beprovided between the light-emitting layer and a second electrode servingas a cathode. That is, by such a structure, layers for controllingmovement of carriers can be provided on both sides of the light-emittinglayer. Then, carriers are recombined in portions separated from theelectrodes, the portions being on both sides of the light-emittinglayer, which is preferable. As a result, by control of the movement ofcarriers on both sides of the light-emitting layer, change in morphologyor effect of crystallization, aggregation, or the like is furtherreduced. Accordingly, a light emitting layer having a long lifetime canbe obtained, in which change with time is hardly caused and reduction inlight emission efficiency with time is hardly caused.

Embodiment Mode 3

In this embodiment mode, a mode of a light-emitting element in which aplurality of light-emitting units of the present invention are stacked(hereinafter, this light-emitting element is referred to as astacked-type element) will be explained with reference to FIG. 9. Thelight-emitting element is a stacked-type light-emitting elementincluding a plurality of light-emitting units between a first electrodeand a second electrode. Each of the light-emitting units may have asimilar structure to that of the EL layer shown in Embodiment Mode 1 andEmbodiment Mode 2. That is, the light-emitting elements shown inEmbodiment Mode 1 and Embodiment Mode 2 are each a light-emittingelement having one light-emitting unit, whereas the light-emittingelement explained in this embodiment mode has a plurality oflight-emitting units.

In FIG. 9, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. The first electrode 501 and the second electrode 502 maybe similar to the electrodes shown in Embodiment Mode 1. The firstlight-emitting unit 511 and the second light-emitting unit 512 may have,either the same structure or a different structure, which may be similarto those shown in Embodiment Mode 1 and Embodiment Mode 2.

A charge generating layer 513 includes a composite material of anorganic compound and a metal oxide. The composite material of an organiccompound and a metal oxide is the composite material shown in EmbodimentMode 1 and includes the organic compound and a metal oxide such asvanadium oxide, molybdenum oxide, or tungsten oxide. As the organiccompound, various compounds such as an aromatic amine compound, acarbazole derivative, aromatic hydrocarbon, and a high molecularcompound (oligomer, dendrimer, polymer, or the like) can be used. As theorganic compound, it is preferable to use the organic compound that hasa hole transporting property and has hole mobility of 10⁻⁶ cm²/Vs orhigher. However, other substances than these may also be used as long asthe hole transporting property is higher than the electron transportingproperty. The composite material of the organic compound and the metaloxide can realize low-voltage driving and low-current driving because ofa superior carrier injecting property and carrier transporting property.

Alternatively, the charge generating layer 513 may be formed bycombination of a layer including the composite material of the organiccompound and the metal oxide with a layer formed using another material.For example, a layer including the composite material of the organiccompound and the metal oxide may be combined with a layer including acompound selected from substances with electron-donating properties anda compound having a high electron transporting property. Moreover, alayer including the composite material of the organic compound and themetal oxide may be combined with a transparent conductive film.

In any case, it is acceptable as long as the charge generating layer 513interposed between the first light-emitting unit 511 and the secondlight-emitting unit 512 injects electrons into one of theselight-emitting units and holes to the other when a voltage is applied tothe first electrode 501 and the second electrode 502. For example, inFIG. 9, it is acceptable as long as the charge generating layer 513injects electrons into the first light-emitting unit 511 and holes tothe second light-emitting unit 512 in a case where a voltage is appliedso that potential of the first electrode is higher than that of thesecond electrode.

This embodiment mode explains the light-emitting element having twolight-emitting units. However, the present invention can be similarlyapplied to a light-emitting element in which three or morelight-emitting units are stacked. When the charge generating layer isprovided between the pair of electrodes so as to partition the pluralityof light-emitting units like the light-emitting element of thisembodiment mode, a long lifetime element in a high luminance region canbe realized while keeping current density low. When the light-emittingelement is applied for lighting, voltage drop due to resistance of anelectrode material can be reduced, thereby achieving homogeneous lightemission in a large area. Moreover, a light-emitting device of low powerconsumption, which can be driven at a low voltage, can be realized.

When light-emitting units have different emission colors, light emissionof desired color can be obtained as a whole light-emitting element. Forexample, in the light-emitting element having two light-emitting units,when emission color of the first light-emitting unit and emission colorof the second light-emitting unit are complementary colors, alight-emitting element emitting white light as a whole light-emittingelement can be obtained. It is to be noted that “complementary color”refers to a relation between colors which become achromatic color bybeing mixed. That is, white light emission can be obtained by mixture oflight obtained from substances emitting the light of complementarycolors. Similarly, also in a light-emitting element including threelight-emitting units, white light emission can be similarly obtained asa whole light-emitting element in a case where emission color of a firstlight-emitting unit is red, emission color of a second light-emittingunit is green, and emission color of a third light-emitting unit isblue, for example.

This embodiment mode can be combined with another embodiment mode asappropriate.

Embodiment Mode 4

In this embodiment mode, a light-emitting device that has alight-emitting element of the present invention will be explained. Inthis embodiment mode, a light-emitting device that has a light-emittingelement of the present invention in a pixel portion will be explainedwith reference to FIG. 10A and FIG. 10B. FIG. 10A is a top view showinga light-emitting device, and FIG. 10B is a cross-sectional view takenalong a line A-A′ and a line B-B′ in FIG. 10A.

In FIG. 10A, reference numeral 601 denotes a driver circuit portion(source driver circuit), 602 denotes a pixel portion, and 603 denotes adriver circuit portion (gate driver circuit), each of which is shown bya dotted line. Reference numeral 604 denotes a sealing substrate, 605denotes a sealing member, and 607 denotes a space surrounded by thesealing member 605. A lead wiring 608 is a wiring for transmittingsignals to be input to the source driver circuit 601 and the gate drivercircuit 603, and receives a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (Flexible PrintedCircuit) 609 that is an external input terminal. Although only the FPCis shown here, a printed wiring board (PWB) may be attached to the FPC.The light-emitting device in this specification includes not only thelight-emitting device itself, but also a state where the FPC or the PWBis attached to the light-emitting device.

Next, the cross-sectional structure is explained with reference to FIG.10B. Although the driver circuit portion and the pixel portion areformed over an element substrate 610, FIG. 10B shows the source drivercircuit 601 that is the driver circuit portion and one pixel in thepixel portion 602. The source driver circuit 601 includes a CMOS circuitformed by combining an N-channel TFT 623 and a P-channel TFT 624.Alternatively, the driver circuit may be formed using a CMOS circuit, aPMOS circuit, or an NMOS circuit. In this embodiment mode, theintegrated driver circuit that is formed over the substrate is shown;however, the driver circuit is not necessarily formed over the substrateand may be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels each having aswitching TFT 611, a current controlling TFT 612, and a first electrode613 that is electrically connected to a drain of the current controllingTFT 612. An insulator 614 is formed to cover an edge portion of thefirst electrode 613. Here, the insulator 614 is formed using a positivephotosensitive acrylic resin film.

In order to improve coverage, an upper edge portion or a lower edgeportion of the insulator 614 is formed so as to have a curved surfacewith curvature. For example, when positive photosensitive acrylic isused for the insulator 614, it is preferable that only the upper edgeportion of the insulator 614 have a curved surface with a radius ofcurvature of 0.2 to 3 μm. Either a negative type that becomes insolublein an etchant by light irradiation or a positive type that becomessoluble in an etchant by light irradiation can be used as the insulator614.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. As a material used for the first electrode 613, variousmetals, alloys, electroconductive compounds, or a mixture thereof can beused. When the first electrode is used as an anode, it is preferable touse, among those materials, a metal, an alloy, an electroconductivecompound, a mixture thereof, or the like with a high work function (workfunction of 4.0 eV or higher). For example, it is possible to use asingle layer film of an indium oxide-tin oxide film containing silicon,an indium oxide-zinc oxide film, a titanium nitride film, a chromiumfilm, a tungsten film, a zinc film, a platinum film, or the like. It isalso possible to use a stacked film such as a stack of a film containingtitanium nitride and a film mainly containing aluminum or a three-layerstructure of a titanium nitride film, a film mainly containing aluminum,and a titanium nitride film. With the stacked structure, a low wiringresistance, favorable ohmic contact, and a function as an anode can beachieved.

The EL layer 616 is formed by various methods such as an evaporationmethod using an evaporation mask, an inkjet method, and a spin coatingmethod. The EL layer 616 includes the layer for controlling movement ofcarriers and the light-emitting layer shown in Embodiment Modes 1 and 2.As another material included in the EL layer 616, a low molecularcompound or a high molecular compound (including oligomer or dendrimer)may be used. As the material for the EL layer, not only an organiccompound but also an inorganic compound may be used.

As a material used for the second electrode 617, various metals, alloys,electroconductive compounds, or a mixture thereof can be used. When thesecond electrode is used as a cathode, it is preferable to use, amongthose materials, a metal, an alloy, an electroconductive compound, amixture thereof, or the like with a low work function (a work functionof 3.8 eV or lower). For example, an element that belongs to Group 1 or2 of the periodic table, that is, an alkali metal such as lithium (Li)or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium(Ca), or strontium (Sr), an alloy containing these (magnesium-silver,aluminum-lithium), or the like can be given. When light generated in theEL layer 616 is transmitted through the second electrode 617, the secondelectrode 617 can be formed using a stack of a metal thin film and atransparent conductive film (indium oxide-tin oxide (ITO), indiumoxide-tin oxide containing silicon or silicon oxide, indium oxide-zincoxide (IZO), indium oxide containing tungsten oxide and zinc oxide(IWZO), or the like).

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealing member 605, a light-emittingelement 618 is provided in the space 607 surrounded by the elementsubstrate 610, the sealing substrate 604, and the sealing member 605.The space 607 may be filled with filler, and may be filled with an inertgas (such as nitrogen and argon), the sealing member 605, or the like.

An epoxy-based resin is preferably used for the sealing member 605. Thematerial preferably allows as little moisture and oxygen as possible topenetrate. As a material for the sealing substrate 604, a plasticsubstrate made of FRP (Fiberglass-Reinforced Plastics), PVF (polyvinylfluoride), polyester, acrylic, or the like can be used besides a glasssubstrate or a quartz substrate.

As described-above, the light-emitting device that has a light-emittingelement of the present invention can be obtained. The thus obtainedlight-emitting device of the present invention has a light-emittingelement with a long lifetime; therefore, the light-emitting device has along lifetime. In addition, the light-emitting device of the presentinvention has a light-emitting element with high light emissionefficiency; therefore, a light-emitting device with reduced powerconsumption, which can emit light with high luminance, can be obtained.

As described above, in this embodiment mode, an active matrixlight-emitting device in which operation of a light-emitting element iscontrolled by a transistor is explained. Alternatively, a passive matrixlight-emitting device may also be used, which operates thelight-emitting element without providing elements for driving such as atransistor, and a structure of that case is shown in FIGS. 11A and 11B.FIGS. 11A and 11B respectively show a perspective view and across-sectional view of a passive matrix light-emitting device which ismanufactured by application of the present invention. It is to be notedthat FIG. 11A is a perspective view showing a light-emitting device, andFIG. 11B is a cross-sectional view taken along a line X-Y in FIGS. 11Aand 11B. In FIG. 11A, an EL layer 955 is provided between an electrode952 and an electrode 956 over a substrate 951. An edge portion of theelectrode 952 is covered with an insulating layer 953. Further, apartition layer 954 is provided over the insulating layer 953. A sidewall of the partition layer 954 slopes so that a distance between oneside wall and the other side wall becomes narrow toward a substratesurface. In other words, a cross section of the partition layer 954 inthe direction of a short side is trapezoidal, and a base (a side facingin the same direction as a plane direction of the insulating layer 953and in contact with the insulating layer 953) is shorter than an upperside (a side facing in the same direction as the plane direction of theinsulating layer 953 and not in contact with the insulating layer 953).By the partition layer 954 being provided in this manner, defects of thelight-emitting element due to static electricity or the like can beprevented. Also in the passive matrix light-emitting device, by thelong-lifetime light-emitting element of the present invention beingincluded, a long-lifetime light-emitting device can be obtained.Further, by the light-emitting element of the present invention withhigh light emission efficiency being included, a light-emitting devicewith reduced low power consumption can be obtained.

Embodiment Mode 5

In this embodiment mode, an electronic device of the present invention,which includes the light-emitting device shown in Embodiment Mode 4 aspart thereof, will be explained. An electronic device of the presentinvention has a light-emitting element shown in Embodiment Modes 1 to 3and a display portion with a long lifetime. Further, since thelight-emitting element has high light emission efficiency, a displayportion with reduced low power consumption can be obtained.

Examples of the electronic device manufactured by using thelight-emitting device of the present invention are as follows: a camerasuch as a video camera or a digital camera, a goggle type display, anavigation system, a sound reproducing device (a car audio system, anaudio component, or the like), a computer, a game machine, a portableinformation terminal (a mobile computer, a mobile phone, a mobile gamemachine, an electronic book, or the like), an image reproducing deviceprovided with a recording medium (specifically, a device for reproducinga recording medium such as a digital versatile disc (DVD) and having adisplay device for displaying the image), and the like. FIGS. 12A to 12Dshow specific examples of these electronic devices.

FIG. 12A shows a television device according to the present invention,which includes a chassis 9101, a support base 9102, a display portion9103, a speaker portion 9104, a video input terminal 9105, and the like.In this television device, the display portion 9103 includeslight-emitting elements similar to those explained in Embodiment Modes 1to 3, which are arranged in matrix. The light-emitting element has afeature that the lifetime is long. Since the display portion 9103including the light-emitting element also has the similar feature, thistelevision device has a feature that the lifetime is long. That is, atelevision device that can withstand long-time use can be provided.Further, since a light-emitting element has high light emissionefficiency, a television device having a display portion of which powerconsumption is reduced can be obtained.

FIG. 12B shows a computer according to the present invention, whichincludes a main body 9201, a chassis 9202, a display portion 9203, akeyboard 9204, an external connection port 9205, a pointing device 9206,and the like. In this computer, the display portion 9203 includeslight-emitting elements similar to those explained in Embodiment Modes 1to 3, which are arranged in matrix. The light-emitting element has afeature that the lifetime is long. Since the display portion 9203including the light-emitting element also has the similar feature, thiscomputer has a feature that the lifetime is long. That is, a computerthat can withstand long-time use can be provided. Further, since alight-emitting element has high light emission efficiency, a computerhaving a display portion of which power consumption is reduced can beobtained.

FIG. 12C shows a mobile phone according to the present invention, whichincludes a main body 9401, a chassis 9402, a display portion 9403, anaudio input portion 9404, an audio output portion 9405, an operation key9406, an external connection port 9407, an antenna 9408, and the like.In this mobile phone, the display portion 9403 includes light-emittingelements similar to those explained in Embodiment Modes 1 to 3, whichare arranged in matrix. The light-emitting element has a feature thatthe lifetime is long. Since the display portion 9403 including thelight-emitting element also has the similar feature, this mobile phonehas a feature that the lifetime is long. That is, a mobile phone thatcan withstand long-time use can be provided. Further, since alight-emitting element has high light emission efficiency, a mobilephone having a display portion of which power consumption is reduced canbe obtained.

FIG. 12D shows a camera according to the present invention, whichincludes a main body 9501, a display portion 9502, a chassis 9503, anexternal connection port 9504, a remote control receiving portion 9505,an image receiving portion 9506, a battery 9507, an audio input portion9508, an operation key 9509, an eye-piece portion 9510, and the like. Inthis camera, the display portion 9502 includes light-emitting elementssimilar to those explained in Embodiment Modes 1 to 3, which arearranged in matrix. The light-emitting element has a feature that thelifetime is long. Since the display portion 9502 including thelight-emitting element also has the similar feature, this camera has afeature that the lifetime is long. That is, a camera that can withstandlong-time use can be provided. Further, since a light-emitting elementhas high light emission efficiency, a camera having a display portion ofwhich power consumption is reduced can be obtained.

As described above, the applicable range of the light-emitting device ofthe present invention is so wide that the light-emitting device can beapplied to electronic devices in various fields. By using thelight-emitting device of the present invention, an electronic devicehaving a long-lifetime display portion that can withstand long-time usecan be provided. Further, an electronic device having a display portionof which power consumption is reduced can be obtained.

Alternatively, the light-emitting device of the present invention canalso be used as a lightning device. One mode using the light-emittingelement of the present invention as a lightning device will be explainedwith reference to FIG. 13. FIG. 13 shows an example of a liquid crystaldisplay device using the light-emitting device of the present inventionas a backlight. The liquid crystal display device shown in FIG. 13includes a chassis 901, a liquid crystal layer 902, a backlight 903, anda chassis 904, and the liquid crystal layer 902 is connected to a driverIC 905. The light-emitting device of the present invention is used forthe backlight 903, and current is supplied through a terminal 906.

By using the light-emitting device of the present invention as thebacklight of the liquid crystal display device, a long-lifetimebacklight can be obtained. The light-emitting device of the presentinvention is a lightning device with planar light emission, and can havea large area. Therefore, the backlight can have a large area, and aliquid crystal display device having a large area can be obtained.Furthermore, the light-emitting device of the present invention is thinand consumes low power; therefore, a thin shape and low powerconsumption of a display device can also be achieved. Furthermore, sincethe light-emitting element has high light emission efficiency, alight-emitting device that can emit light with high luminance can beobtained. Furthermore, since the light-emitting device of the presentinvention has a long lifetime, a liquid crystal display device with along lifetime can be obtained.

FIG. 14 shows an example of using the light-emitting device, to whichthe present invention is applied, as a table lamp that is a lightingdevice. A table lamp shown in FIG. 14 has a chassis 2001 and a lightsource 2002, and the light-emitting device of the present invention isused as the light source 2002. Since the lifetime of the light-emittingdevice of the present invention is long, the lifetime of the table lampis also long.

FIG. 15 shows an example of using the light-emitting device, to whichthe present invention is applied, as an indoor lighting device 3001.Since the light-emitting device of the present invention can have alarger area, the light-emitting device of the present invention can beused as a lighting device having a large area. Further, since thelifetime of the light-emitting device of the present invention is long,the light-emitting device of the present invention can be used as along-lifetime lighting device. A television device 3002 according to thepresent invention as described in FIG. 12A is placed in a room in whichthe light-emitting device, to which the present invention is applied, isused as the indoor lighting device 3001. Thus, public broadcasting andmovies can be watched. In such a case, since each of the devices has thelong lifetime, the number of replacing the lighting device and thetelevision device can be reduced, whereby the environmental load can bereduced.

Embodiment 1

In this embodiment, a light-emitting element of the present inventionwill be specifically explained with reference to FIG. 16. A structuralformula of an organic compound used in Embodiment 1 and Embodiment 2will be shown below.

[Chemical Formula]

(Manufacture of Light-Emitting Element 1)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate 2201 by a sputtering method, whereby a firstelectrode 2202 was formed. Note that the thickness of the firstelectrode 2202 is 110 nm, and the electrode area is 2 mm×2 mm.

Next, the substrate having the first electrode 2202 was fixed to asubstrate holder provided in a vacuum evaporation apparatus in such away that the surface of the first electrode 2202 faced downward, andthen the pressure was reduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated on the first electrode 2202,whereby a layer 2211 including a composite material was formed. Theevaporation rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Note that the co-evaporation method isan evaporation method in which evaporation is performed using aplurality of evaporation sources at the same time in one treatmentchamber.

Next, a hole transporting layer 2212 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) tothe thickness of 10 nm by an evaporation method using resistanceheating. After that, a light-emitting layer 2213 was formed over thehole transporting layer 2212. The light-emitting layer 2213 was formedby co-evaporating 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole(abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the evaporationrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA: 2PCAPA).

Further, a layer 2214 for controlling movement of carriers was formed byco-evaporating tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)and 2PCAPA to a thickness of 10 nm over the light-emitting layer 2213.Here, the evaporation rate was controlled so that the weight ratio ofAlq to 2PCAPA could be 1:0.003 (=Alq:2PCAPA). After that, an electrontransporting layer 2215 was formed by depositing bathophenanthroline(abbreviation: BPhen) to the thickness of 30 nm over the layer 2214 forcontrolling movement of carriers by an evaporation method usingresistance heating.

Next, an electron injection layer 2216 was formed by depositing lithiumfluoride (LiF) to the thickness of 1 nm over the electron transportinglayer 2215. Finally, a second electrode 2204 was formed by depositingaluminum to the thickness of 200 nm by an evaporation method usingresistance heating. Consequently, a light-emitting element 1 was formed.The light-emitting element 1 of the present invention obtained throughthe above-described process was put into a glove box containing anitrogen atmosphere so that the light-emitting element was sealed fromatmospheric air.

Then, the operating characteristics of the light-emitting element 1 weremeasured. Note that the measurement was conducted at a room temperature(atmosphere kept at 25° C.). FIG. 17 shows the current density vs.luminance characteristics of the light-emitting element 1. FIG. 18 showsthe voltage vs. luminance characteristics of the light-emitting element1. FIG. 19 shows the luminance vs. current efficiency characteristics ofthe light-emitting element 1. FIG. 20 shows the emission spectrum of thelight-emitting element 1 with a current supply of 1 mA. FIG. 21 showsthe result of the light-emitting element 1, in which a continuouslighting test by constant current driving was conducted with the initialluminance set at 5000 cd/m² (the vertical axis indicates the relativeluminance on the assumption that 5000 cd/m² is 100%).

The emission color of the light-emitting element 1 was located at theCIE chromaticity coordinates of (x=0.28, y=0.65) at the luminance of5000 cd/m², and green emission which derives from 2PCAPA was obtained.In addition, the current efficiency and driving voltage of thelight-emitting element 1 at the luminance of 5000 cd/m² were 18 cd/A and3.6 V, respectively. Further, when a continuous lighting test of thelight-emitting element 1 by constant current driving was conducted withthe initial luminance set at 5000 cd/m², 93% of the initial luminancewas maintained even after 260 hours. Thus, it was proved that thelight-emitting element 1 has a long lifetime.

(Manufacture of Reference Light-Emitting Element 2)

Next, for the purpose of comparison, a reference light-emitting element2 without 2PCAPA included in the layer for controlling movement ofcarriers in the above-described light-emitting element 1 was formed. Themanufacturing method will be described below. First, indium oxide-tinoxide containing silicon oxide was deposited over a glass substrate by asputtering method, whereby a first electrode was formed. Note that thethickness of the first electrode is 110 nm, and the electrode area is 2mm×2 mm.

Next, the substrate having the first electrode was fixed to a substrateholder provided in a vacuum evaporation apparatus in such a way that thesurface of the first electrode faced downward, and then the pressure wasreduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated on the first electrode, wherebya layer including a composite material was formed. The evaporation ratewas controlled so that the thickness of the layer including a compositematerial could be 50 nm and the weight ratio of NPB to molybdenum(VI)oxide could be 4:1 (=NPB:molybdenum oxide). Note that the co-evaporationmethod is an evaporation method in which evaporation is performed usinga plurality of evaporation sources at the same time in one treatmentchamber. Then, a hole transporting layer was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by an evaporation method using resistance heating.

Next, a light-emitting layer 2213 was formed over the hole transportinglayer. The light-emitting layer 2213 was formed by co-evaporating9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA)and N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the evaporationrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA: 2PCAPA).

Further, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) wasdeposited to be a thickness of 30 nm over the light-emitting layer. Thatis, unlike the light-emitting element 1, a layer without including2PCAPA, which includes only Alq, was formed. Thereafter, an electrontransporting layer was formed by depositing bathophenanthroline(abbreviation: BPhen) to a thickness of 30 nm over the layer includingonly Alq by an evaporation method using resistance heating.

Then, an electron injection layer was formed by depositing lithiumfluoride (LiF) to a thickness of 1 nm over the electron transportinglayer. Finally, a second electrode was formed by depositing aluminum toa thickness of 200 inn by an evaporation method using resistanceheating. Consequently, the reference light-emitting element 2 wasformed. The reference light-emitting element 2 obtained through theabove-described process was put into a glove box containing a nitrogenatmosphere so that the light-emitting element was sealed fromatmospheric air.

Thereafter, the operating characteristics of the referencelight-emitting element 2 were measured. Note that the measurement wasconducted at a room temperature (atmosphere kept at 25° C.). Theemission color of the reference light-emitting element 2 was located atthe CIE chromaticity coordinates of (x=0.29, y=0.64) at the luminance of5000 cd/m²; the current efficiency of the reference light-emittingelement 2 was 18 cd/A; and it exhibited green emission which derivesfrom 2PCAPA similarly to the light-emitting element 1. However, when acontinuous lighting test was conducted with the initial luminance set at5000 cd/m², luminance has decreased to 75% of the initial luminanceafter 260 hours as shown in FIG. 21. Thus, it was found that thereference light-emitting element 2 has a shorter lifetime than thelight-emitting element 1. Therefore, it was proved that a long-lifetimelight-emitting element can be obtained by applying the presentinvention.

Embodiment 2

In this embodiment, a light-emitting element of the present inventionwill be specifically explained with reference to FIG. 16.

(Manufacture of Light-Emitting Element)

First, indium oxide-tin oxide containing silicon oxide was depositedover a glass substrate 2201 by a sputtering method, whereby a firstelectrode 2202 was formed. Note that the thickness of the firstelectrode 2202 is 110 nm, and the electrode area is 2 mm×2 mm.

Next, the substrate having the first electrode 2202 was fixed to asubstrate holder provided in a vacuum evaporation apparatus in such away that the surface of the first electrode 2202 faced downward, andthen the pressure was reduced to about 10⁻⁴ Pa. Then,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated on the first electrode 2202,whereby a layer 2211 including a composite material was formed. Theevaporation rate was controlled so that the thickness of the layer 2211could be 50 nm and the weight ratio of NPB to molybdenum(VI) oxide couldbe 4:1 (=NPB:molybdenum oxide). Note that the co-evaporation method isan evaporation method in which evaporation is performed using aplurality of evaporation sources at the same time in one treatmentchamber.

Next, a hole transporting layer 2212 was formed by depositing4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) to athickness of 10 nm by an evaporation method using resistance heating.Subsequently, a light-emitting layer 2213 was formed over the holetransporting layer 2212. The light-emitting layer 2213 was formed byco-evaporating 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole(abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) to a thickness of 30 nm. Here, the evaporationrate was controlled so that the weight ratio of CzPA to 2PCAPA could be1:0.05 (=CzPA: 2PCAPA).

Further, a layer 2214 for controlling movement of carriers was formed byco-evaporating tris(8-quinolinolato)aluminum(III) (abbreviation: Alq)and Coumarinne 30 to a thickness of 10 nm over the light-emitting layer2213. Here, the evaporation rate was controlled so that the weight ratioof Alq to Coumarin 30 could be 1:0.005 (=Alq:Coumarin 30). After that,an electron transporting layer 2215 was formed by depositingbathophenanthroline (abbreviation: BPhen) to a thickness of 30 nm overthe layer 2214 for controlling movement of carriers by an evaporationmethod using resistance heating.

Then, an electron injection layer 2216 was formed by depositing lithiumfluoride (LiF) to a thickness of 1 nm over the electron transportinglayer 2215. Finally, a second electrode 2204 was formed by depositingaluminum to a thickness of 200 nm by an evaporation method usingresistance heating. Consequently, a light-emitting element 3 was formed.The light-emitting element 3 of the present invention obtained throughthe above-described process was put into a glove box containing anitrogen atmosphere so that the light-emitting element was sealed fromatmospheric air. Then, the operating characteristics of thelight-emitting element 3 were measured. Note that the measurement wasconducted at a room temperature (atmosphere kept at 25° C.).

FIG. 22 shows the current density vs. luminance characteristics of thelight-emitting element 3. FIG. 23 shows the voltage vs. luminancecharacteristics of the light-emitting element 3. FIG. 24 shows theluminance vs. current efficiency characteristics of the light-emittingelement 3. FIG. 25 shows the emission spectrum of the light-emittingelement 3 with a current supply of 1 mA. The emission color of thelight-emitting element 3 was located at the CIE chromaticity coordinatesof (x=0.28, y=0.65) at the luminance of 5000 cd/m², and green emissionwhich derives from 2PCAPA was obtained. In addition, the currentefficiency and driving voltage of the light-emitting element 3 at theluminance of 5000 cd/m² were 18 cd/A and 3.5 V, respectively.

Characteristics Measurement Example

In this embodiment, reduction reaction characteristics oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), and Coumarin 30, which are used for the layerfor controlling movement of carriers in the light-emitting elements 1and 2 manufactured in Embodiments 1 and 2, were observed by cyclicvoltammetry (CV) measurement. Further, the LUMO levels of Alq, 2PCAPA,and Coumarin 30 were determined from the measurement results. Note thatan electrochemical analyzer (ALS model 600A or 600C, manufactured by BASInc.) was used for the measurement.

As for a solution used in the CV measurement, dehydrateddimethylformamide (DMF, manufactured by Sigma-Aldrich Inc., 99.8%,catalog No. 22705-6) was used for a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, manufactured by Tokyo Chemical Industry Co.,Ltd., catalog No. T0836), which is a supporting electrolyte, wasdissolved in the solvent such that the concentration oftetra-n-butylammonium perchlorate was 100 mmol/L. Further, the object tobe measured was also dissolved in the solvent such that theconcentration thereof was 1 mmol/L. In addition, a platinum electrode (aPIE platinum electrode, product of BAS Inc.) was used as a workingelectrode; a platinum electrode (a VC-3 Pt counter electrode (5 cm),manufactured by BAS Inc.) was used as an auxiliary electrode; and anAg/Ag⁺ electrode (an RE5 nonaqueous solvent reference electrode,manufactured by BAS Inc.) was used as a reference electrode. Note thatthe measurement was conducted at a room temperature (20 to 25° C.).

(Calculation of the Potential Energy of the Reference Electrode withRespect to the Vacuum Level)

First, potential energy (eV) of the reference electrode (Ag/Ag⁺electrode) used in Embodiment 2 with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation-reduction potential offerrocene in methanol is +0.610 V [vs. SHE] with respect to a standardhydrogen electrode (Reference: Christian R. Goldsmith et al., J. Am.Chem. Soc., Vol. 124, No. 1, pp. 83-96, 2002). On the other hand, whenthe oxidation-reduction potential of ferrocene in methanol wascalculated using the reference electrode used in Embodiment 2, theresult was +0.20 V [vs. Ag/Ag⁺]. Therefore, it was found that thepotential energy of the reference electrode used in Embodiment 2 waslower than that of the standard hydrogen electrode by 0.41 [eV].

Here, it is also known that the potential energy of the standardhydrogen electrode with respect to the vacuum level is −4.44 eV(Reference: Toshihiro Ohnishi and Tamami Koyama, High molecular ELmaterial, Kyoritsu shuppan, pp. 64-67). Accordingly, the potentialenergy of the reference electrode used in Embodiment 2 with respect tothe vacuum level could be determined to be −4.44−0.41=−4.85 [eV].

Measurement Example 1 Alq

In Measurement Example 1, the reduction reaction characteristics of Alqwere observed by cyclic voltammetry (CV) measurement. The scan rate wasset at 0.1 V/sec. FIG. 26 shows the measurement result. Note that themeasurement of the reduction reaction characteristics was conducted bythe steps of: scanning the potential of the working electrode withrespect to the reference electrode in ranges of (1) −0.69 V to −2.40 V,and then (2) −2.40 V to −0.69 V.

As shown in FIG. 26, it can be seen that a reduction peak potentialE_(pc) is −2.20 V and an oxidation peak potential E_(pa) is −2.12 V.Therefore, half-wave potential (an intermediate potential between E_(pc)and E_(pa)) can be determined to be −2.16 V. This shows that Alq can bereduced by an electrical energy of −2.16 V [vs. Ag/Ag⁺], and this energycorresponds to the LUMO level. Here, the potential energy of thereference electrode used in this measurement example with respect to thevacuum level is −4.85 [eV] as described above. Therefore, the LUMO levelof Alq can be determined to be −4.85−(−2.16)=−2.69 [eV].

Measurement Example 2 2PCAPA

In Measurement Example 2, the reduction reaction characteristics of2PCAPA were observed by cyclic voltammetry (CV) measurement. The scanrate was set at 0.1 V/sec. FIG. 27 shows the measurement result. Notethat the measurement of the reduction reaction characteristics wasconducted by the steps of: scanning the potential of the workingelectrode with respect to the reference electrode in ranges of (1) −0.41V to −2.50 V, and then (2) −2.50 V to −0.41 V. As shown in FIG. 27, itcan be seen that a reduction peak potential E_(pc) is −2.21 V and anoxidation peak potential E_(pa) is −2.14 V.

Therefore, half-wave potential (an intermediate potential between E_(pc)and E_(pa)) can be determined to be −2.18 V. This shows that 2PCAPA canbe reduced by an electrical energy of −2.18 V [vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, the potential energy of thereference electrode used in this measurement example with respect to thevacuum level is −4.85 [eV] as described above. Therefore, the LUMO levelof 2PCAPA can be determined to be −4.85−(−2.18)=−2.67 [eV]. Note thatwhen the LUMO levels of Alq and 2PCAPA which were calculated in theabove-described manner are compared, it can be found that the LUMO levelof 2PCAPA is lower than that of Alq by as much as 0.02 [eV]. Therefore,it is quite advantageous to use Alq and 2PCAPA for the layer forcontrolling movement of carriers.

Measurement Example 3 Coumarin 30

In Measurement Example 3, the reduction reaction characteristics ofCoumarin 30 were observed by cyclic voltammetry (CV) measurement. Thescan rate was set at 0.1 V/sec. FIG. 28 shows the measurement result.Note that the measurement of the reduction reaction characteristics wasconducted by the steps of: scanning the potential of the workingelectrode with respect to the reference electrode in ranges of (1) −0.21V to −2.30 V, and then (2) −2.30 V to −0.21 V. As shown in FIG. 28, itcan be seen that a reduction peak potential E_(pc) is −2.07 V and anoxidation peak potential E_(pa) is −1.99 V.

Therefore, half-wave potential (an intermediate potential between E_(pc)and E_(pa)) can be determined to be −2.03 V. This shows that Coumarin 30can be reduced by an electrical energy of −2.03 V [vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, the potential energy of thereference electrode used in this measurement example with respect to thevacuum level is −4.85 [eV] as described above. Therefore, the LUMO levelof Coumarin 30 can be determined to be −4.85−(−2.03)=−2.82 [eV]. Notethat when the LUMO levels of Alq and Coumarin 30 which were calculatedin the above-described manner are compared, it can be found that theLUMO level of Coumarin 30 is lower than that of Alq by as much as 0.13[eV]. Therefore, it is quite advantageous to use Alq and Coumarin 30 forthe layer for controlling movement of carriers.

This application is based on Japanese Patent Application serial no.2006-184350 filed in Japan Patent Office on Jul. 4, 2006 and JapanesePatent Application serial no. 2006-327609 filed in Japan Patent Officeon Dec. 4, 2006, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A light-emitting element comprising: a firstelectrode; a second electrode; a light-emitting layer between the firstelectrode and the second electrode, the light-emitting layer comprisinga first substance having a light-emitting property; a first layerbetween the second electrode and the light-emitting layer, the firstlayer comprising: a second substance having a carrier transportingproperty; and a third substance for reducing the carrier transportingproperty of the second substance; and a second layer between the secondelectrode and the first layer, the second layer comprising a fourthsubstance having a carrier transporting property, wherein the secondsubstance is different from the fourth substance.
 2. A light-emittingelement according to claim 1, wherein a difference between a wavelengthof the highest peak of an emission spectrum of the third substance and awavelength of the highest peak of an emission spectrum of the firstsubstance is within 30 nm.
 3. A light-emitting element according toclaim 1, wherein an emission color of the third substance and anemission color of the first substance are similar colors.
 4. Alight-emitting element according to claim 1, wherein the first layer isin contact with the light-emitting layer.
 5. A light-emitting elementaccording to claim 1, wherein a thickness of the first layer is greaterthan or equal to 5 nm and less than or equal to 20 nm.
 6. Alight-emitting element according to claim 1, wherein a proportion ofweight of the second substance is greater than a proportion of weight ofthe third substance in the first layer.
 7. A light-emitting elementaccording to claim 1, wherein the carrier is an electron, and wherein adifference between a lowest unoccupied molecular orbital level of thesecond substance and a lowest unoccupied molecular orbital level of thethird substance is less than 0.3 eV.
 8. A light-emitting elementaccording to claim 1, wherein the carrier is an electron, wherein thesecond substance is a metal complex, and wherein the third substance isan aromatic amine compound.
 9. A light-emitting element according toclaim 1, wherein the carrier is a hole, and wherein a difference betweena highest occupied molecular orbital level of the second substance and ahighest occupied molecular orbital level of the third substance is lessthan 0.3 eV.
 10. A light-emitting element according to claim 1, whereinthe carrier is a hole, wherein the second substance is an aromatic aminecompound, and wherein the third substance is a metal complex.
 11. Alight-emitting element according to claim 1, wherein the light-emittinglayer further comprises a fifth substance having a carrier transportingproperty, and wherein a proportion of weight of the fifth substance isgreater than a proportion of weight of the first substance in thelight-emitting layer.
 12. A light-emitting element according to claim11, wherein structures of the second substance and the fifth substanceare different from each other.
 13. A light-emitting element according toclaim 1, further comprising a third layer between the first layer andthe light-emitting layer, the third layer comprising a fifth substancehaving a carrier transporting property.
 14. A light-emitting elementaccording to claim 13, wherein the third layer is in contact with thelight-emitting layer and the first layer.
 15. A light-emitting elementaccording to claim 1, further comprising a third layer between the firstelectrode and the light-emitting layer, the third layer comprising: afifth substance having a carrier transporting property; and a sixthsubstance for reducing the carrier transporting property of the fifthsubstance, wherein a proportion of weight of the fifth substance isgreater than a proportion of weight of the sixth' substance in the thirdlayer.
 16. A light-emitting device comprising the light-emitting elementaccording to claim
 1. 17. An electronic device comprising a displayportion comprising the light-emitting element according to claim
 1. 18.A lighting device comprising the light-emitting element according toclaim
 1. 19. A light-emitting element comprising: an anode; a cathode; alight-emitting layer between the anode and the cathode, thelight-emitting layer comprising a first substance having alight-emitting property; a first layer between the cathode and thelight-emitting layer, the first layer comprising: a second substancehaving an electron transporting property; and a third substance having ahole transporting property; and a second layer between the cathode andthe first layer, the second layer comprising a fourth substance havingan electron transporting property, wherein the second substance isdifferent from the fourth substance.
 20. A light-emitting elementaccording to claim 19, wherein a difference between a wavelength of thehighest peak of an emission spectrum of the third substance and awavelength of the highest peak of an emission spectrum of the firstsubstance is within 30 nm.
 21. A light-emitting element according toclaim 19, wherein an emission color of the third substance and anemission color of the first substance are similar colors.
 22. Alight-emitting element according to claim 19, wherein the first layer isin contact with the light-emitting layer.
 23. A light-emitting elementaccording to claim 19, wherein a thickness of the first layer is greaterthan or equal to 5 nm and less than or equal to 20 nm.
 24. Alight-emitting element according to claim 19, wherein a proportion ofweight of the second substance is greater than a proportion of weight ofthe third substance in the first layer.
 25. A light-emitting elementaccording to claim 24, wherein a difference between a lowest unoccupiedmolecular orbital level of the second substance and a lowest unoccupiedmolecular orbital level of the third substance is less than 0.3 eV. 26.A light-emitting element according to claim 24, wherein the secondsubstance is a metal complex, and wherein the third substance is anaromatic amine compound.
 27. A light-emitting element according to claim24, wherein the light-emitting layer has an electron transportingproperty.
 28. A light-emitting element according to claim 24, wherein,when a magnitude of a dipole moment of the second substance is P₁ and amagnitude of a dipole moment of the third substance is P₂, P₁/P₂≧3 issatisfied.
 29. A light-emitting element according to claim 24, whereinthe light-emitting layer further comprises a fifth substance having anelectron transporting property, and wherein a proportion of weight ofthe fifth substance is greater than a proportion of weight of the firstsubstance in the light-emitting layer.
 30. A light-emitting elementaccording to claim 29, wherein structures of the second substance andthe fifth substance are different from each other.
 31. A light-emittingelement according to claim 19, further comprising a third layer betweenthe first layer and the light-emitting layer, the third layer comprisinga fifth substance having an electron transporting property.
 32. Alight-emitting element according to claim 31, wherein the third layer isin contact with the light-emitting layer and the first layer.
 33. Alight-emitting element according to claim 19, further comprising a thirdlayer between the anode and the light-emitting layer, the third layercomprising: a fifth substance having a hole transporting property; and asixth substance having an electron transporting property, wherein aproportion of weight of the fifth substance is greater than a proportionof weight of the sixth substance in the third layer.
 34. Alight-emitting device comprising the light-emitting element according toclaim
 19. 35. An electronic device comprising a display portioncomprising the light-emitting element according to claim
 19. 36. Alighting device comprising the light-emitting element according to claim19.
 37. A light-emitting element comprising: an anode; a cathode; alight-emitting layer between the cathode and the anode, thelight-emitting layer comprising a first substance having alight-emitting property; a first layer between the anode and thelight-emitting layer, the first layer comprising: a second substancehaving a hole transporting property; and a third substance having anelectron transporting property; and a second layer between the anode andthe first layer, the second layer comprising a fourth substance having ahole transporting property, wherein the second substance is differentfrom the fourth substance.
 38. A light-emitting element according toclaim 37, wherein a difference between a wavelength of the highest peakof an emission spectrum of the third substance and a wavelength of thehighest peak of an emission spectrum of the first substance is within 30nm.
 39. A light-emitting element according to claim 37, wherein anemission color of the third substance and an emission color of the firstsubstance are similar colors.
 40. A light-emitting element according toclaim 37, wherein the first layer is in contact with the light-emittinglayer.
 41. A light-emitting element according to claim 37, wherein athickness of the first layer is greater than or equal to 5 um and lessthan or equal to 20 nm.
 42. A light-emitting element according to claim37, wherein a proportion of weight of the second substance is greaterthan a proportion of weight of the third substance in the first layer.43. A light-emitting element according to claim 42, wherein a differencebetween a highest occupied molecular orbital level of the secondsubstance and a highest occupied molecular orbital level of the thirdsubstance is less than 0.3 eV.
 44. A light-emitting element according toclaim 42, wherein the second substance is an aromatic amine compound,and wherein the third substance is a metal complex.
 45. A light-emittingelement according to claim 42, wherein the light-emitting layer has ahole transporting property.
 46. A light-emitting element according toclaim 42, wherein, when a magnitude of a dipole moment of the secondsubstance is P₁ and a magnitude of a dipole moment of the thirdsubstance is P₂, P₁/P₂≦0.33 is satisfied.
 47. A light-emitting elementaccording to claim 42, wherein the light-emitting layer furthercomprises a fifth substance having a hole transporting property, andwherein a proportion of weight of the fifth substance is greater than aproportion of weight of the first substance in the light-emitting layer.48. A light-emitting element according to claim 47, wherein structuresof the second substance and the fifth substance are different from eachother.
 49. A light-emitting element according to claim 37, furthercomprising a third layer between the first layer and the light-emittinglayer, the third layer comprising a fifth substance having a holetransporting property.
 50. A light-emitting element according to claim49, wherein the third layer is in contact with the light-emitting layerand the first layer.
 51. A light-emitting element according to claim 37,further comprising a third layer between the cathode and thelight-emitting layer, the third layer comprising: a fifth substancehaving an electron transporting property; and a sixth substance having ahole transporting property, wherein a proportion of weight of the fifthsubstance is greater than a proportion of weight of the sixth substancein the third layer.
 52. A light-emitting device comprising thelight-emitting element according to claim
 37. 53. An electronic devicecomprising a display portion comprising the light-emitting elementaccording to claim
 37. 54. A lighting, device comprising thelight-emitting element according to claim 37.